In reply to iusedtoclimb:
I think the rule of thumb is to drop the intensity but not the volume too much. So ease off the hill repeats and hard efforts 2-3 weeks out but don't ease down and too much overall mileage or you risk losing fitness.
I'm doing a 100 miler on 1 June and am planning to do my last big run on 20th May (we've got a local 46km/2500m race here which I plan to just take really easy) and then really ease off after that. I'm still doing weekly long runs now but probably won't do any hard workout sessions after this week.
Ultimately listen to your body though.