Some estimate the for every pound lost, you gain 2 seconds a mile. However, diet affects running in a variety of ways, not only weight/time. Going vegan (yes, difficult) will enable you to recover more quickly as well as to lose weight. Vegan: no meat, fish, dairy. Try it, perhaps by cutting out meat first and then seeing how you feel. I was skeptical at first, but would never go back. Being leaner and lighter will not only produce a direct time benefit; it will also, indirectly, allow you to train more and with greater quality. It will also make you less injury-prone. If you are worried about ridicule, don't tell people you are doing it; I never use the "v" word. I just say plant-based. Veganism isn't a cult; it is about health and potentially about fitness. Read Scott Jurek's Eat & Run or anything by Dean Ornish, Caldwell Esselstyn and his son Rip Esselstyn. Caldwell got gold in rowing many years ago, and Rip is a serious triathlete. And, of course, Jurek is a legend. My wife, a tennis player, dropped nearly 100 cholesterol points in three months of a flexible veganism; she has milk in her coffee. And, of course, she lost weight. Vegans are always thin.