I wonder what your times for shorter races are. You said that you have 5 20 mile runs and a 60 mile week; my guess is that 4 hours might well be too modest of a goal for you.
For a conservative goal, take your recent 10K time and multiply it by 5, or take your recent 1/2 marathon time, multiply by 2 and add 15-20 minutes.
You sound much better prepared than I ever was! I'd consider the following: your pace should feel "too easy" for the first 10 miles; if you are not getting antsy you are probably going a bit too fast.
Anyway, be confident as it is evident that you've done your preparation!
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