When I have the heart-rate monitor with me, it's really easy to stick to that zone. I shoot for a target heart rate of about 150, and I can go a long time. Last time I tried it was 7 miles, and I stopped because I'd looped around and was close to home. I set it up with RunKeeper so that it gives me a heart-rate report every minute, and I just tweak my effort a little bit to keep in right around the target.
