Baseado no comentário de Tadaen Sylvermane, percebi que havia algo meio corrompido no arquivo de fontes originalmente listado. Decidi substituir isso por um arquivo de fontes padrão para minha distribuição. Eu fui para: link , selecionei os repositórios padrão para meu país e distribuição, gerou um novo arquivo de fontes, substituí meu arquivo antigo com o arquivo recém-gerado e executou uma atualização do apt.
Como mostrado, algumas URLs ainda estavam faltando:
ejspeiro@Eduardo-Alienware-14:/etc/apt$ sudo apt-get update
Ign file: InRelease
Get:1 file: Release.gpg [819 B]
Get:2 file: Release [574 B]
Ign file: Translation-en_US
Ign file: Translation-en
Hit http://ppa.launchpad.net trusty InRelease
Ign http://us.archive.ubuntu.com trusty InRelease
Hit http://ppa.launchpad.net trusty/main amd64 Packages
Get:3 http://us.archive.ubuntu.com trusty-security InRelease [65.9 kB]
Hit http://ppa.launchpad.net trusty/main armhf Packages
Hit http://ppa.launchpad.net trusty/main i386 Packages
Hit http://ppa.launchpad.net trusty/main Translation-en
Get:4 http://us.archive.ubuntu.com trusty-updates InRelease [65.9 kB]
Get:5 http://us.archive.ubuntu.com trusty-proposed InRelease [65.9 kB]
Hit http://us.archive.ubuntu.com trusty-backports InRelease
Hit http://us.archive.ubuntu.com trusty Release.gpg
Get:6 http://us.archive.ubuntu.com trusty-security/main amd64 Packages [588 kB]
Hit http://us.archive.ubuntu.com trusty-security/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-security/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-security/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-security/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/main Translation-en
Hit http://us.archive.ubuntu.com trusty-security/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-security/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-security/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/main Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/universe Translation-en
Hit http://us.archive.ubuntu.com trusty Release
Hit http://us.archive.ubuntu.com trusty/main Sources
Hit http://us.archive.ubuntu.com trusty/restricted Sources
Hit http://us.archive.ubuntu.com trusty/universe Sources
Hit http://us.archive.ubuntu.com trusty/multiverse Sources
Hit http://us.archive.ubuntu.com trusty/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty/main Translation-en
Hit http://us.archive.ubuntu.com trusty/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty/universe Translation-en
Err http://us.archive.ubuntu.com trusty-security/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-security/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-security/universe armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-security/multiverse armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/universe armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/multiverse armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-proposed/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-proposed/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-proposed/universe armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-proposed/multiverse armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/main armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/restricted armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/universe armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/multiverse armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty/universe armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty/multiverse armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Ign http://us.archive.ubuntu.com trusty/main Translation-en_US
Ign http://us.archive.ubuntu.com trusty/multiverse Translation-en_US
Ign http://us.archive.ubuntu.com trusty/restricted Translation-en_US
Ign http://us.archive.ubuntu.com trusty/universe Translation-en_US
Fetched 785 kB in 5min 41s (2,302 B/s)
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
No entanto, quando executada a instalação do aptitude, tudo correu bem. Além disso, o sistema de atualização está começando a me solicitar algumas atualizações agora.
Quanto ao motivo pelo qual o link gera arquivos de origem com URLs desatualizados me confunde. Além disso, eu não entendo por que, se este é um LTS que deve ser suportado até 2019, então por que repositórios não podem ser encontrados?
De qualquer forma, estou satisfeito com este procedimento e seu resultado, e é por isso que decidi publicá-lo como uma resposta à minha pergunta original.