I think running is the best workout for me, and sometimes I can run up to 45 minutes (4 miles). Other times, though, I have trouble running for 10 minutes. I don't understand why there is such a difference in my endurance. Also, once I've run one day I can't really run again that well for a week, at least. Is there anything I'm doing wrong? Is there something good to eat before you run that will help with endurance?