All I know is what works for me, so I'll share that. I never stretch before or after I ride or run. However, I always start at a very comfortable pace until I feel that I've warmed up sufficiently. When I go to spin classes, I do go through whatever the cool down routine that the instructor presents. Also, I take yoga one or two times a week. I have had problems in the past with tight IT bands, so I occasionally use a foam roller, or tiger tail to work out the kinks.

It sounds like you have come a very long way, and getting those last 10 miles in are going to be much easier than the first 10 you did. Just listen to your body, figure out your nutrition & hydration needs, and you'll get to 75.