Chris - I was in exactly your position last year!
Training for Berlin 2006 - developed BAD shin splints 5 weeks out and had no choice but to stop running for over 3 weeks. I'd just got back to some short running about 11 days before, but had maintained my fitness by swimming like someone possessed in the meantime.
I had been hoping for a sub 3:15 time and had been doing over 60 miles per week, with around 7 20+ milers. In the end I ran 3:23-ish which I was pleased with given the circumstances. I really don't think I lost a lot of fitness....I think the main reasons I was slower than I'd hoped for were that I was actually still injured, and the temperature was 26 degrees!
By the way, I'm sure my shin splint recovery ended up being far longer than it would have otherwise been, due to my stubborness in doing the marathon regardless (oh yes, and the Great North Run the following week)....are you sure you are recovered enough?