Under 4 Hour Marathon?

Over the last few weeks, I've been looking at what it takes to run a sub 4 hour marathon. Effectively to run under 4 hours, you need to average around the 9 minute mile mark. I've not managed this in any run more than about 10-11 miles so far, however my current pace for a 10k is well under that. According to my research to achieve a sub 4 hour marathon you need to be running 20-25 miles per week already!!! I'll probably manage around 15 miles this week, having done 2 x 3.1 earlier in the week and an easy 2 miles yesterday. I'll probably do 3 tomorrow and a longer run on Sunday.

Being really optimistic as I usually am, I think under 4 hours is achievable although a friend of mine who I consider to be extremely fit and a regular marathon runner has never broken the 4 hour mark!!