What does your long-term race "training" look like before the training plan begins?

I'm interested in running a marathon in a little over a year. I've run 3 half-marathons, improving my time each race, and think I will be ready for the challenge then.

Looking up marathon training plans (or plans of any length really) they are generally max 20 weeks, and meant to be followed leading right up to the race. So what do you do if your goal is something that's much further out?

Or maybe another way to ask: those who run marathons semi-regularly, what are you doing in the in-between? Are you slowly ramping up your mileage over a series of months, or are you consistently running very long runs the whole year? Do you keep your MPW relatively low until it's time to start the 16-20-week training program? Curious about what more experienced runners are doing!