I didn't run today until dusk after the sun had gone down. I had a hard time breathing, and my energy level was low. The main problem, I think, was that I hadn't eaten much during the day, and I was concerned about not having enough energy for the run. I ate two peanut butter sandwiches about half an hour before I left for the run so I would have some energy. I really knew I shouldn't do that, because I've done similar things with bad results. But, I was hungry and yielded to the temptation to eat. I did my planned distance, but I did a lot of walking during the last part of the distance.
I normally eat a few handfuls of dry cereal before I run, and they are fine because they are easily digested. Apparently peanut butter isn't easily digested. In addition, peanut butter has a lot of protein, and it isn't a good idea to eat protein during a run (save the protein for the first half hour after you finish). Live and learn; I'm doing plenty of living, but I'm not sure how much learning I'm doing :)