Did you watch the fortunes of last year's FLM RW beginners? I think it was Kerry (?). You can.
There are issues here. One is, you were once much better at it. The second is the fear of sniggerers.
Going back and starting at a much lower point than you left a sport, or any skill, is one of the very Hard things in life.
Facing the sniggerers also takes more courage than most folks realize. But standing tall in the presence of sniggerers is actually a terrific sensation. Go ahead, snigger. But I have my trainers on, and I am doing something about it, and some day you may be fat and fifty and I'll be fit and seventy and laughing at you. Practice it in the mirror before you leave. Get used to it, and it feels great!
My equivalent is coming last in races. The feeling "yeah, think what you like, but I did it" always puts me on top of a wave. Every time I find a reason not to run, and don't run because of it, I feel Seriously low.
Give yourself heaps of credit for attempting it. In terms of brownie points, you getting out the door in trainers for 30 minutes counts 10 times what the speedy boys and girls achieve on a 22 mile run.
Sign up for the marathon, say you are going to do it even if you crawl or roll it, and you are going to be hugely proud of that finisher's medal.
Go for it girl!
(I'm 52, not getting any younger, and have the distinction that until October this year, my pb in a half marathon was also one in which I came last in! I am improving.)