Hi Julia,
It sounds to me that the pressure of it all is starting to catch up with you as it does a lot of people this time of year and missing out whilst ill was probably only adding to the situation.
Personally if I was in your shoes and I have experienced something similar then I would resume training but go to a level back a couple of weeks before you had to stop. I would stop worrying about times and just get some miles into your legs and focus entirely on getting yourself fit enough to get around. You aren't going to win the thing so why not just enjoy it.
Hopefully by taking some of the pressure off of yourself you can enjoy it a bit more. Here is a thought, If this is your first marathon and you crawled the entire course on hands and knees you'd still have done a marathon, recorded a PB and got the medal to prove it 
You may also find that relaxing about the situation and just taking it as it comes may restore a bit of zip into your training and you may find yourself on the start line in better shape then you can imagine now. Good luck with it all