Racing two marathons in three weeks is manageable if your training has good volume and is consistent. The key would be to spend three days after Blackpool running each day, but only for 20 mins on soft terrain, you're legs will feel stiff, even terrible, but the gentle running should get rid of it and getting really good sleep is essential! Try and get some normal training in two weeks after and then taper leading in to Manchester. Just be careful.
Agree with Grendel3, Plimsolls on Eyeballs Out is a brilliant read about Jim Peters, as is Rob Hadgraft's other book Arthur Nerwton, Tea With Mr Newton. Last To First by Spedding is great and in the index page there's extracts of his training leading up to ther Olympic marathon, worth the price of the book on its own. There was a book out not long ago about Gordon Pirie, The Impossible Hero which was good.
Lore Of Running is a classic, if not heavy read, but it covers all you'd want to know and more.
It's great that Uk marathons are doing well. I just wished i'd predicted this would sell out earlier. Again, if anyone has a number and for whatever reason is unable to do it could they contact me and I'll pay the fee: email@example.com.