You can do this! I got my first place in the London Marathon in the October and ran in April. I went to the gym a few times a week but wasn’t a runner. In the April I did the marathon, took me just over 6 hours as it was the hottest one ever so I slowed down and walked for parts to cope with it. It changed me forever. Doing all that training and then getting to achieve the London marathon (which is spectacular) just changed something in my brain chemistry.
An average 16 week marathon training block starts at Christmas for London and you need to be up for doing 2-3 mid week runs, plus a weekend long run every single week from then until the marathon. The first “long” run for the beginners plan is usually 10km or 7 miles. Therefore you’ve got between now and Christmas to get running that far.
Agree with other posters that your priority is couch25km and also starting regular strength training. Get into the routine because it only gets harder! Get fitted for proper running shoes (and expect to have to get fitted again before the race) and make sure you have a decent sports bra.
If you have a charity spot, think about fundraising and the running schedule! Try and prioritise fundraising this side of Christmas or in January/early Feb. Weeks 8-13 of the marathon training plan will likely being really tough and you don’t want a cake sale/raffle/pub quiz to add to your stress! Weeks 15-16 you will be easing off the running so a good time to do some fundraising to capitalise on the build up!
It’s going to be hard! I’m currently 6 weeks out from the NY marathon, running some really big miles and can’t believe how much my legs ache and how hungry I am alllllll the time and how much sleep I need! But that’s the beauty of it- you’re going to do something really really hard but the sense of achievement is incredible!
You have got this and you absolutely will do it!!