Not able to run in the heat?

Finally the weather starts getting better! today it was about 90 degrees which is considered scorching in Chicago. I am starting my training for over the summer for xc in the fall. Last year i came pretty close to going to state, so this summer I'm really going to push myself to get better. But today with the heat, I wasn't even able to run a solid 3 miles, my training for today actually was supposed to be 6. During the run, I just got really heated and my legs felt heavy. I had to stop a couple to times to walk. What should I do? I'm really frustrated that I wasn't able to get in my mileage. Thanks!