Pacing problems

Whatever is the matter with me? I set out today to run 8 miles at a 7:30 pace, but couldn't bring myself to start off that slowly, and by the time I'd done 4 miles at 6:50 pace, that was it. I was toast. Clearly, I'd have stood a better chance of finishing the 8 if I'd paced better. But the course is so hilly, it's hard for me to tell if I'm going too fast or too slow. Only 11 months until the Labor Day 5-miler in Ocean City ... I better be in shape for that.