FWIW, I know not a lot!, but it does sound like some of this is a mental challenge ... am I right in thinking you were getting quite stressed about this a few weeks back? or am I confusing you with someone else? I think you DO need to ease up on yourself, if you get closer to the day and genuinely think 'sod it, I just can't do this', then yes you've lost some money but at the end of the day???? When I do any kind of endurance training I don't bang out the same long miles week after week, I always have an easy shorter week every 4th week, I vary the distance of the long run so I'd do perhaps when building it up ... 10, 12, 12, 10, 15, 12, 18, 10, 15, 20, 15, 10 and so on, building up to the distance I think my longest run should be - if that longest run isn't until a month out from the actual race then that is fine by me!and put a run in of some sort the day after the long run ... that run I make ANY kind of distance, whatever fits in really, sometimes it's 3-5 miles sometimes it's 8-10.
Life is stressful enough at times without stressing about the things you are doing for 'fun'.