Since I've switched to a vegan diet, I'm amazed at the difference. I never get sore, although I do eventually get fatigued. Recovery after walking is much quicker than it used to be on diet that included meat.

I've given my subconscious a new set of ground rules to work by. It's simply to walk in a way that won't cause injury. Then I don't think about my pace or technique at all. This has increased my pace without increasing fatigue.

There is a certain amount of satisfaction to "pushing yourself like you've never pushed yourself before," but it is not the solution to long term success if your goal is to ride fast without expensive medical procedures along the way.

Excess protein is converted to fat, and along the way leeches calcium from the bones causing osteoporosis in the distant future. It is almost impossible to get too little protein with a varied diet of fruits, vegetables, whole grains and nuts. A person only needs about 20% of their calories from protein, and that number has been doubled twice for a factor of safety since the original studies.

In order to pull a hamstring, you are most likely pedaling too slowly (pushing.) This can cause knee problems and severe back problems. They used to recommend pedaling at around 90 rpm give or take the conditions at the time. If you want to learn to "spin" get yourself a set of bicycle rollers like these: http://amzn.com/B000BT8VPG. They will seem impossible to ride at first, but they will make you a smoother and faster rider with about 15 minutes of practice a day. Do not get rollers that attach to the rear wheel.

As for your hamstring, if you were in Colorado, I'd recommend some green medicine, but since you aren't, I'd recommend 2 weeks of rest. Your muscles will be stronger after two weeks than they are now.