So, in spite of my attempts to wean prior to Milk Dud's second birthday, we are still breastfeeding. It isn't necessarily every day and isn't nursing like a newborn, but I was curious as to the effects of breastfeeding with long distance running.
First of all, breastfeeding requires lots of maternal fluid intake. Running does as well, particularly during the summer. If you are a breastfeeding mother who is doing any sort of exercise, you should be very aware of your fluid intake. Feeling thirst is past the point where you should be drinking. I would encourage all breastfeeding runners to carry water with them while nursing.
I could one real study on breastfeeding and running. That can be found here, but I will summarize it for you. The baby was fussy, but there was no difference in lactic acid concentration in pre- and post-run milk. The researchers concluded that more study was needed. Obviously, if they only had one sample point, the reason could be from anything. I found this great informational site from Kelly Mom where they encouraged exercise and breastfeeding. They have suggestions that if the baby refuses to nurse, you rinse off your breasts or postpone nursing for 30 minutes. I also found this great blog post where she talked about how her breast pump was her most important running accessory ;)
2004 Olympic Bronze Medalist and American record holder for the marathon Deena Kastor breastfed and started back to her training regimen during those months. You can read an interview with her here.
I amazingly didn't need to pump during my relay race, but I'm nearing the end of my breastfeeding journey. Women train and breastfeed all the time. You just have to be aware of your needs and those of your baby.