exhaustion

I'm having a hard time keeping my eyes open. And I'm supposed to run 6 miles in an hour. Hmmmm...how am I going to muster up the energy? For the past two days I've run 3 miles in the terrible heat/humidity and then gone straight to boot camp at the gym where I participated in hard-core exercises for an hour. I think 2 days in a row of that has put me over the edge of exhaustion. I was thinking maybe I could put the run off until tomorrow but I don't have any time to run tomorrow at all. And my last day of boot camp is tomorrow and I don't want to do both in one day again. Usually when I feel this way I just tell myself to take it slowly on the run and do a run/walk if I have to. But I have dinner plans immediately following the run and have no extra time to spare. I guess what I will do is take it slowly and if I don't feel the energy pick up at all, I will run 5 miles instead of 6. It's early enough in the training (3.5 weeks into it) that reducing one run by one mile is not going to make a difference. I just need to make sure I don't consistently do this during the training or I won't feel ready on marathon day.