What do you think marathon training is?
Run 100 mile weeks.. your legs are tired constantly, but you come out of that phase fitter than you went in..
Its all very short term, symptomatic of the now society we live in, resting may help in 5 days time, but not in 5 weeks time. I'm not against rest, but for a young guy one day a week should be enough assuming the usual training loads. Older people, obviously more R and R.
But steady 10 milers, 1 min or so off race pace will really help change your physiology, trim you down, increase you running efficiency. For anyone only running 20-30 miles a week more quality running miles = an improvement in 95% of cases... not 20 min miling fell runs.. they maybe enjoyable but physiological benefit will be minimal compared to a flatter trail run with nice runnable hills.