I have no problems on my walks , just in the gym . I go around 8am or so and just hit a wall about a hour in , I need to figure this out .^ I have no problem doing 3-5 miles in the morning w/o breakfast- this is in Zone 2 (relatively easy); harder or longer miles I need some calories before hand (and if long enough, during). Ditto on going to the gym and lifting, fasting is no bueno for me- I want a good meal under my belt