Timothy and BlazingSaddles have it right. In the beginning when we started riding all miles built up our VO2 and leg strength. Little junk miles then IMHO.
After a while we plateau unless we work harder such as doing high intervals and mixing up on our rides, like different intervals. Just like when our bodies sleep our muscles need recovery time after a hard effort to rest so the muscles can get stronger. Muscles only get stronger with hard and varying efforts then recovery.
Some coaches believe middle Heart Zones (forget exact level), neither make us stronger nor help us rest. This effort is call no mans zone or otherwise refer to a wasted effort--guess one could call them junk miles. The point is if you want to get stronger we need to ride at varying intensities including recovery times.
So, OldsCool if you are serious about getting ever stronger, best to learn about how to train or hire a trainer as I have and consider a power meter to help measure your effort.
Thanks for asking a great question.