I've been listening to an Audio Book called Primal Endurance:
It basically recommends doing:
1. Slow runs keeping your heartbeat under 180 - your age.
2. Sprint training (proper short hard sprint training, as opposed to tempo or fartlek).
The point seems to be to train our bodies to be less reliant on glycogen and use our ketonic (fat burning) system as our primary source of energy. Or something like that.
It's really for my Missis, she's a fairly decent ultra runner (she's ran 3 this year, including a 100 miler and came first lady in two and fourth in her last).
I could see Primal Endurance working for me (fat 'n' slow), but what about a more accomplished ultra runner?
I'd be interested to hear your views.