I'd say go for it. I've always found it really hard to get myself motivated to go out after a big target race like a marathon.
Having something to do afterwards may seem a good idea. I've also found that when I have dragged myself out after a long hard race, after the initial soreness has worn off, I've flown round my training routes.
I'd take it as a bit of fun, you don't know what to expect, so don't set a real target to push for. Do some fairly gentle runs in-between, don't push in training at all. The training you've done for the marathon should see you round easily. If you struggle, well, it's still nice to get out and do something, if you feel strong a few miles in, well then push along and see what you can do.
On the point about the pros not racing so close to a marathon, you need to remember that they push themselves far closer to their limits than us mere mortals do (even though it feels as if we're trying just as hard). They are also doing it to compete at the highest level (and earn prize money), you can do it just for fun!