John's got it. I would add that you don't want to run for 100 feet and then walk. If you pick up the pace, I suggest you do it for 1/4 mile or more for two reasons. 1. If you run fast for 100 feet then walk for 300 feet, you will piss off the people you just passed as they get stuck behind you on some single track. 2. You will realize that you can run comfortably once you get past the discomfort of the first couple hundred feet. If you have HRM, use it to determine the effort at which you are running. When your HR is maxing out, it might be time to slow down or walk.