I just wanted to chime in, as far as hamstring stretches. It's really, really easy to cheat most hamstring stretches and use your back instead. If you can lie down and stretch your leg up, you can't cheat with your back so easily.

Also wanted to mention that just because you are flexible doesn't mean that your back is necessarily strong everywhere that it needs to be to support you on the bike. There is one muscle group that I have trouble focusing on because of the fusion in my upper back - and that is where I will be aching after a long ride. The more varied your core strengthening exercises, the better!