It is clear that you have had to deal with alot of emotional stress in the past while, and to me, what you are explaining makes perfect sense. I've experienced something like this also that i think i can relate to. You see, my WHOLE life has revolved around my hockey career. It pretty much "defined" who i was to my family ... in my eyes it was what they saw me as. So i went to university this year, and was pretty much expected to play on the varsity hockey team, and i decided i couldn't. I was so scared to let everyone down, that i would let myself down and i let that fear take over me, and i decided not to play. Now everytime i even think about hockey, go anywhere close to the rink or anything i experience severe anxiety.
Now i realise that our cases are different, but i think it has to do with that whole "control" factor. You felt you could control yourself if you participated and trained for this marathon, just as i felt that hockey was the one thing i could control. Now that you've been put in the situation where you have no control over your body, it is natural that you would experience this anxiety because you can't do anything about it. Being out of control of yourself is possibly one of the scaryest things out there. But know, that just because you may not have been able to physically control yourself in this marathon case, you ARE still in control. Instead of focusing on huge goals, such as a marathon, try and focus on the little ones because those accumulated will lead you to the big goal. Everytime you go to exercise, try not to think about "why" you're exercising. Try and stay in the present, focusing on each footstep (if on the tredmill) at a time. Or try and distract yourself by listening to music, or watching the people around you. If you have tried this and you still find yourself not enjoying it, then maybe that is a sign to take a break, or try something new for a while. If you always run when you go to the gym, maybe try going on the bike if you want to exercise. If you find that it doesn't matter what you work out on, then maybe you could try doing something else. Find a new hobby, or if you already have one that you enjoy, try doing that a little more.
I hope this helps ... and if i'm off i really am sorry, but just know that you CAN do whatever you put your mind to, and you do have the strength.
Take care
Jacq