Welcome DB!
Far fewer runners here than there used to be (and, having "graduated", I only stop by occasionally). I do a few halfs each year, but ultras? Hah! In my dreams...
Swimming, stationary bike and weight lifting are ways to maintain fitness during recovery. Your greatest risk of weight gain likely relates to your training habits. Continuing to carb-load as you might've during training, without the counterbalancing exertion, can quickly add weight when sedentary. If fat-adapted, probably not.
Depending on your goals (considering current BMI vs idealized, for example) intermittent fasts can help to maintain, or to lose weight, as desired. Ever bonk or hit the wall? Then's there's also a consideration of converting to full time fat-burning and abandoning carb dependence (can take 4-6 weeks). Have you seen this?
https://www.youtube.com/watch?v=2sqeJEgNuHw