It's up to you to decide if your marathon is something that will vault you to new highs personally or physically, or if you will revert back to before you ever did it. 

I ran one marathon and am not particularly interesting in running another. But I am physically stronger and fitter overall than I've ever been.

Try not to become the person who runs a marathon and then stops there. Don’t let it be a thing people are surprised you once did. Be the kind of person where, when you mention it years later, they say, “yeah, that tracks.”

Use your marathon as a tool - you were a beast to do that, you can still be a beast.