Welcome to the forum, Johnnyb
As Fulmar said, I'm sure you can complete the marathon, just get a decent training plan, work hard and keep at it
A couple of suggestions are probably to get some decent running shoes, if you haven't already, go for distance rather than speed, and try to build up your mileage slowly to avoid injury.
Anyway, well done for losing 2 stone, even if it did come back! Losing weight's really hard to do.