I'd do the other things I already do and know I enjoy. Cross country ski, hike, walk, climb mountains, climb rock/boulders, mountain bike, skijor, backcountry ice skate, ski and mountain bike orienteer, most of those things with my dog. Play disc and tug with my dog. What I like about running is the being outdoors in beautiful places, exploring things and places, feeling fit, moving my body in athletic ways, competition, no competition at other times. There are aspects of all those things in those other things I enjoy.
Things I could get into in another life, but probably not: Modifying and (low level) racing cars, get back into photography, disc golf, dog mushing, vanlife, wingsuit flying.