Im going to be running my second marathon in march and I have a couple of questions about how I should run it.

Im aiming for anything less than 5 hours, which means 11:27 per mile or better. I was thinking that I could start off at 12:00 miles for the first 6-8 miles, then bump it up to 11:30 for the next 15 miles, then pick it up to 10:30-11:00 per mile for the last few miles, to make up for the slower start. I tried it in a 20 mile training run and it worked well. I had plenty of juice left for the last few miles.

will this tactic work for the marathon? Am I better off just aiming for 11:27 all the way? or starting off a little quicker and giving myself room at the end to slow down? I figure this way my heart rate will stay fairly low for the first 8-10 miles, giving me the energy to finish strong, which is better than overdoing it at the beginning.

2nd question: during my shorter runs I find that I can keep my heartrate around 150 (Im 27, so this is where I want to be) while running those first few miles at 11:00 or better. my heart rate slowly creeps up after the first few miles. am I better off sticking to 11:30 or 12:00 per mile, even though my heart rate will be lower than my acceptable range of 150-155, or should I run a little quicker and allow my heart rate to be closer to 155?

In other words, will my heart rate stay lower longer if I run a little slower, giving me considerably more energy in the latter miles of the race, or should I run a little quicker, taking advantage of the lower heart rate at the start of the race to cut a few minutes off my time?

3rd question: I find that I can run at a consistant pace and have a heartrate that varies by 10-20 bpm, mostly based on what Im thinking about. If my thoughts drift for a few minutes, then I look at my monitor, I can find that Im running at 150bpm. then, as soon as I look at my monitor, I become aware of my breathing and heart rate again, and my heart rate will jump to 160-165 within 1 few seconds.

Is it normal for heartrate to vary so much, being completely mental? do you have any advice for how to avoid this?

sorry for the lengthy explanations :) thanks for any advice.