I read something yesterday in Jeff Galloway's blog that caught my attention. He advocates taking more frequent but short walking breaks. Specifically, doing 30 seconds of running and 30 seconds of walking for the whole run. The purpose, I think, is to take frequent but short walking breaks to prevent fatigue, thus keeping ones energy level high. I've been running for about 1/4 mile at the beginning of my run, and then, as my run progresses, running shorter segments with walks in between. My signal that I should stop running and walk for a couple hundred feet is that my legs feel tired. As my run progresses, my spurts of running get shorter, until at the end I'm only running about 50 feet before I stop to walk.
I'm going to try Jeff's suggestion during my run on Saturday. By taking walking breaks more often, my spurts of running will be shorter, and (hopefully) my legs won't get as tired.