Dan A
yes a hard 20 takes more out and often means a worse marathon. I don't see a problem at slower than marathon pace or maybe just on marathon pace if you are experienced and very fit but I too went through a period of running very good 20 mile races and four to six weeks later some very underwhelming marathons.
I think they are useful training exercises for pacing and race simulation but maybe it's best for you to run around 2:10 and just target marathon pace for second half or last five