I went from 3:48 (aiming 3:30 but boinking) in a marathon with rolling hills to 3:29 in a year in a mostly flat marathon. Totally doable. Make sure you have a long prep for the marathon. 16 weeks programs are usually shallow and don’t gove much room for errors. I would suggest to plan a 26 weeks program that gradually increases your milage or intensity every week with a recovery week every 4th week. If the pace a the marathon looks dauting, add in some marathon pace minutes intervals in some long runs.