I agree, blocks a marathon pace is a good idea. Things like easy first 10M at MP + 30s then second 10M @ MP. Or my favourite 26M with miles 15-20 @ MP - 10s per miles and 20-25 @ MP. Loads of a variations, but getting used to running MP on tired legs helps everytime.
Also I'm interested at the peak mileage of 21M, I tend to complete 5 or more at 22+ and tend not to run out of gas, just a thought