10 to 12 weeks of 35 to 45miles and 10,000ft+ Feb to May or Mar to Jun and you'll be fine. Don't worry too much what you're doing in the week either (or in Nov or Dec). Swiss Toni on here had a theory you could get round on one run per week and I concur if it was a long (8hr+) run on the high fells. Burn out / breakdown / boredom are risks