Heart Rate Running

[Sweder]

This maybe an old chestnut for the RC thinktank - apologiesif this is well trodden ground.

Met a bloke last night who raved about the heart rate method of fitness training. He'sa pretty fit ex Army chap so I listened intently. The formula goes something like:

Heart Rate: 220 minus your age X 75% = ideal training heart rate.
You walk/ run to get your HR up to this magical figure but you don't exceed it - if you do you slow down until you're back to the magic number. He reckons this ensures fat and not muscle is burned, and the long term effects are excellent for marathon runners.

Nutshell - anyone tried this/ concur?
I seem to remember reading about HR stuff in one of the diaries (too lazy/ overhung to search).
If so, did it work for you?
Finally, any definitive reading recommended on the subject?

I'm intrigued. Seems my current regime may be doing more harm than good.
I'd be happy to sacrifice short term time goals for long term benefits.