The first time I ever heard someone talk about stretching before exercise..... many, many years ago. It was because it was to help with injury due to hyperextension or over extension of ones leg joints. So for a runner or competition walker, it might be a good thing. However for cycling, if your saddle is at the proper height, the pedals will keep you from hyperextending your knees or other joints.

I see no purpose in warming up prior to a ride. Unless you are about to do some actual racing with others on a bicycle. In that case, it might be good to do 30 minutes or so of warm up on a bike or trainer. Sometimes it takes 60 to almost 90 minutes for my legs to get loose and ready power up steep hills at my best times.

As for stretching, if you want to do it, that's fine. Don't do it in the middle of the trail entrance as some runners do here.