If you have already done 3 then you know what it takes. To avoid injury you need to build up slower and/or lose the lard first. If you don't really want to do it no amount of pep talks will convince you. If it's the difference between getting around and doing a reasonable time there are a lot of plans available.
Kickstarted winter training with Cornish marathon at the weekend. Very slow (3;53!) but as I had not run over 10 miles since May that was deliberate. Well did a 16 mile 5 days before to test whether it would even be possible to do the full. Amazed it went so well, quick 5 miles this morning and all seems fine. Now just got to get the speed up! Much prefereing this colder weather to run in!