I've just read this book:
http://www.amazon.co.uk/Big-Book-End...8471195&sr=1-1
And have been at it for approx 4 weeks. Normally I would be training very slowly at this time of year, but at my "MAF" heart rate of 145 BPM I can run just over 11 min miles before I need to slow down.
Incredibly slow running is what it feels like, but the idea is as your body learns to burn fat for fuel you get to run faster at this lower heart rate. Its interesting stuff, the book is not very well referenced though and mostly annecdotal but there are a lot of annecdotes. There is also quite a bit of woo science at the end of the book, but I'm keeping an open mind and am going to try this technique for a couple more months.