I warm up with a 15% grade easy, lol

kidding aside, biking or not, I usually do a set of sit-ups either traditional or straight legged (I alternate days) a set up push-ups and if you can a set of pull-ups. For the legs I do full bent squats into calf raises.

Now how many you do depends on you, I usually just cut it in half of what my max is but usually 10-12 is enough maybe more than needed, the trick for me is, all the reps I do I do them very slowly. The more you keep whatever muscle flexed the more blood will go there and warm you up. It usually only takes me 10 mins.

As far as stretches go, that all depends on you as well. I dont stretch at the beginning but rather at my first or second stop light. This works for me, not everyone, I found that stretching will actually tire you out unnecessarily in the beginning but if I stretch the first 5-10 mins into an excercise not only do I get more range of motion but a better stretch. Like I said it works for me, I've been doing it this way with every excercise since I was 22, Now 27 and still serving in the USMC. Good luck.