Perhaps marathon running just isn't your cup of tea? How about targetting a 5/10k and getting into an appropriate training plan? If not, how about mixing it up with some other exercise and working towards a triathalon? It may also be that having finished your marathon your body needs a period of recuperation with mental fatigue hence your lack of motivation afterall you must have been motivated since marathon training takes a lot of commitment. So on that basis, congratulations! Give yourself a big pat on the back and a smug look, you've earned it!
Don't knock yourself either, celebrate your achievements and set a new goal, good luck!