I'm on week 8 of Wegovy, lifting three days weekly with heavier weights each week. A bit of light cardio too, but muscle gain is the focus (with 79 lbs still to lose, so recomp is possible, adding muscle as fat drops). I can't say if it'd affect your marathon ability. Before lifting, I did around five miles a day on the elliptical and it didn't bother me, yet I've had zero bad side effects, so yours may differ. Anyway, the shot has a one week half-life, so if it interfered you could lower the dose or stop and miss at most a couple workouts before it clears. Low stakes compared to the potential benefits.