Good work on running you first marathon! Yeah - that's a nice wall you run into at 18 miles, eh?
So, don't sweat it, it can take a few weeks (or more sometimes) to get over a marathon. You have a tired body, deep tissue damage and even though you might not feel residual pain - your body needs to recover. It's fine to go out for runs, but they should be shorter, and slower and yes, you will feel like you are not a 'marathon fit' runner! Don't worry it'll all subside in the next few weeks.
I usually take a week or 2 where it's obvious I've just run long, then a week or so where my muscles an ache 'deep' inside if I try to go too fast or too long. So feel very happy you ran your marathon, let your body recover and don't think you need to train through this bad feeling - that is a recipe for injury and woes!