However, if your goal is to succeed at longer distance running (and by this, I mean runs of at least the half-marathon length, then you would stand to benefit by slowing your pace down to where your heart rate is around 65-70% of your maximum. It is at this intensity where your slow-twitch fibers are utilized, and the more that happens, the more your long-distance endurance improves. This is, in fact, part of the reason for the success of elite marathoners--they get faster by incorporating lots of miles at a slow pace. Granted, slow for them is still pretty fast for the rest of us, but virtually anyone, by following this approach, can make similar improvements so that the running pace that gets their heart beating at 65-70% of their max keeps getting faster over time.