I ran london this year, 3:30, picked up my training and was on for a sub 3:20 in Sept (Sadly collapsing due ot dehydration..bugger bugger, bugger) and then picked up my distance work again a couple of weeks later, and have it im my mind that if i run at least one 20+miler per month then i am ready to drop into a marathon or ultra with about 4 weeks notice (i'm not usually any more organised that this to book ahead...)
Last weekend i tackled the Doyen ofd The Downs, my first ultra, and it was wet hard and fun!,m and was pleased with 4:52
Doing training that keeps you up there in terms of fitness, circa 40 week, so having an 8 week gap between events seems, to me a concern about nothing. I've considered the Kent, but I think the tough part will be knowing it's laps, and a drop out is easy. When you;ve got to get back to base, and it's 20 miles away, well...might as well shuffle along hadn't you.
Press the button, you know you want to