I've done something a bit more extreme in the past, training at 300 miles per week and then doing a 5 day, 600 mile event. I did it OK, but wasn't much use for anything for a while afterwards. I'd say that attempting the back to back centuries is a good idea. If you can do that, then 400 miles per week is going to be no problem. One thing to bear in mind, though, is the recovery. Going at touring speed, 60 miles per day doesn't sound like much of a challenge; but it feels tougher after 10 consecutive days in the saddle. So I'd be inclined to plan a route that might involve a slightly higher daily mileage but allows you a couple of rest days.