No cure for upset bowels

This afternoon I went out for a run, intending to go 11.5 miles. The first 40 minutes were uneventful, but after that I began to feel my bowels rebel against me. In other words, I felt like I was going to go number 2 in my shorts. Now, this was not the first time I've had this problem. It's actually becoming quite common, which frustrates me to no end. Running definitely exacerbates the discomfort, and when I stop to walk, I feel fine. Well, that's what I ended up doing today. I managed to run for 2-3 minutes at a time before having to walk. I ended up cutting the run short, only going 9.3 miles.

I am incredibly frustrated with this, especially because otherwise I feel fine. It's rare enough that I can go on a run and avoid blisters, armpit chafing, nipple bleeding, and crotch burn. Not to mention the fact that some days I just feel more tired than others. So when I get a day where I feel great, why do my bowels insist on ruining the run? I've actually considered continuing to run until I soil myself, but the discomfort is simply too great. I can only hope this problem doesn't plague me when I run the JFK 50 miler in a few weeks.