WCW is hillier and is not as 'newbie friendly' as Steve's late September outing- I believe the plan for the Group Perm is a rolling average of 24kph. Of course, if you're not willing/able to ride with the group it's a perm and you can enter & ride at any time. Other 600k perms are also available, including a DIY.
If you sign up for the MK600 then TG will do everything in his not inconsiderable power to get you round.
AIUI he's only had one not make it- and that was Rich when his bike fell apart.
I do however think getting a 400 & a 600 in before the end of September is quite a big ask for a not-that-fast relative newcomer. There aren't that many weekends left, and you will need a decent recovery period (at least a fortnight) between rides. There's less daylight, so a greater proportion of your riding will be at night and almost everyone slows down after dark, meaning that if getting round in time was going to be a struggle it will be an even bigger struggle.
It depends on your psychology- for me, trying and failing would be sufficiently off putting and demoralising that I'd probably wait till next year till my chances were better, having spent the winter plugging away at 200s, and generally training for speed and hills. If you're the kind of person who is happy that they gave their all, even if they don't succeed, then give it a shot. No calendar 400s left though, so that's a lonely 400 you'll have.
