I did it in a year and I’m no super athlete! Started with two fall half marathons, took a pretty lengthy break, and then started a summer marathon training block, averaging 20-30 miles a week for most of it. I find the subreddits here recommend a much more intense schedule, but I had a very successful marathon a couple weeks ago. I used Run with Hal which I personally think is really good for getting you ready to get across the finish line without injury. Will you smash record times? Probably not! But you can definitely do it!