Can do a loop from byres road along great western road then down crow road and along Clarence drive or vice versa - there's only one or two traffic lights you need to stop and and it's not v hilly (minus Clarence drive but I normally stop at the bottom of it) and it's main roads so well lit. Depending where you start/finish it's about 5/6K - it's nice as well to swing by Bingham's pond and do a lap or two round there and back if you don't want to go all the way. Only downside is the byres road section can be busy with pedestrians depending on what time of day you go.
https://preview.redd.it/i87cp5ca4lpb1.jpeg?width=1080&format=pjpg&auto=webp&s=7951d4c5e30bf2b8cf60177f1c62d9c8dbe24a7a |