I stretch every morning just before breakfast, but I never need to do so until I turned 65.

I don't usually warm up. Maybe I ride a little easier the first couple miles is all. That said, it takes me almost an hour of relatively hard riding before everything is functioning normally. If I ride easy, those changes don't happen.

By far the most useful thing you could do is to ride ~4 times/week, every week. Say three 1-hour rides if that's all you have time for, and one longer ride every weekend. Riding will then be even more fun.

A good way to warm up is to stand for a couple minutes, maybe after the first mile. Gets the blood moving.