Well done in getting the running bug
I've got it too, haven't had a run since 24th January, the day before my last surgery and it's properly doing my head in now
My running had improved exponentially since wls, I'm not a better runner, just thinner so my speed and endurance is much increased.
Firstly a marathon is an excellent goal to have. I've done two now. A reality check, it's 42km, 26.2 miles, yes MILES
be very sure that you are up for the 1 -2 years training for this daft thing to do. More than likely you'll be injured many times, it'll be boring running in the rain for hours and hours and hours... Etc. think very carefully if you are to go down this path. Be realistic. It's not about the day of the race, it's the miles you put in day in day out for the previous year.
Running a 5km is an achievement, improving your time is an achievement. If you have a Parkrun near you, getting your 50 runs shirt is an achievement.
10km's are next, then 10 miles, Half MarathonS are the next order of the day. No harm in stopping at any of these great milestones.
Take it easy, get proper trainers, a running club as you have found is a great place to be. Enjoy running, that's the greatest achievement.
A bit of advice on races you enter. Big events are stupidly expensive, why pay the high entry. Running clubs and smaller events are much cheaper and more fun. An exception to this is the Muscular Dystrophy run in Oxford in May, Cambridge in October, and there is another one somewhere else, it's only £20, for the 10km.
Enjoy it. I'm so jealous at the moment as I can't run to I'm healed more, it seems like weeks and weeks....