Here are a few things to look at around the use of a heart rate monitor. They are a great way to measure what is going on with your body regardless of age, weight or level of health. And if you want to burn fat from your body you can't beat it for a way to monitor what you're doing.
Check out this article and this chart (the chart has two pages - read the second page and understand what they're saying!).
http://www.lifetimefitness.com/modul...ning_chart.pdf
http://www.duathlon.com/articles/1460
http://www.active.com/story.cfm?CHEC...STORY_ID=12408
http://www.trifuel.com/triathlon/tri...ate-000574.php
You burn more fat at a slower pace (under your threshold) than you do at a higher pace where you are burning sugar for fuel. When that is gone and you stay at a high pace you r body goes for the muscle for fuel. That's where you heard others talking about the problems with a 700 calorie/day diet. If there's no sugar left and you are cranking to high your body goes for the easiest to process source - muscle. It's easier for your body to process fat at slower tempos.
Anyway, read through all this and think about getting a heart rate monitor. You'll be glad you did if your are serious about weight management and health.