While running, I get 3 miles in and have to have a bowel movement. I have tried going before but doesn't help. What can I do? I have been running about a year and this has been a problem since adding more distance. I have tried going before I run but often

This . This is a common complaint of distance runners. The mechanism is not entirely clear but I have suggested to my patients to use Imodium (loperamide) or another antidiarrheal in a prophylactic manner. I have had great success with this. It usually only takes one or two 15-30 minutes before a run and you will be much more comfortable. I would also suggest staying away from sugar or high electrolyte drinks prior to the run as they can also stimulate bowel movements.
Nutrition. Irritable bowel from running is often due to a nutritional problem. Look at caffeine and fat intake. When in doubt, visit a sports nutritionist that can help you. If its happening all the time, not just when running, time to visit a GI specialist.