Sam: AS a running coach that uses heart rate monitors in my clients training I have found that on race day, depending on what your goals and objectives are, heart rate zones are best used to keep you from going out to fast. If you are running your 20 miler at 75 to 85% with no problems, then that would be a good zone to utilize, may be a lower zone for the first half of the race then move up to a higher zone for the second half, ensuring in a negitive split. Is you first marathon going to be hilly - which would have an impact on heart rate. Also which formula did you use to determine your zones? The old 220 minus your age times a percentage really doesn't provide accurate information that can be of true benefit for the heart monitor trained marathoner.