So I am a runner who is a Vegetarian. Since going veggie, I have had an increase in energy, and have lost some poundage as well which will help with the knees and back.
There have been some good articles on vegetarian diets and running - mostly on www.runnersworld.com. The benifits of becoming a vegetarian far outweigh any cons.
It is quick and easy to prepare most meals, it is nutritous, filling, and quite palatable! You experience new foods that possibly you would not have tried before (tofu for one.)
You should give it a try! Add some new veggies for strength and endurance, as well as to get some more nutritous snacks.
http://www.vegkitchen.com/ has some wonderful recipies for you to try!