I was a runner, but mostly 5k's, never more than a half-marathon. Ultramarathoners tend to be exceptionally lean, for obvious reasons. You don't carry extra weight for 50-100 miles. Most males weigh about 120-130 pounds. I don't care how fit you are, in any physical confrontation, any dad-bod 200-240 pound male will snap any 120 pounder in half, with minimal effort.
I'm 230, and a decade or two removed from any physical efforts (was 185 when I was barely active in my 30s). I have zero fight training, and yet I would not fear for a moment getting in a ring with even professional boxers or MMA guys in the straw weight (115) or flyweight (125) divisions. Most 120# runners offer virtually zero physical threat to any average male. I would show more caution facing off against a typical 7th grade soccer player.
Because I know a couple and have seen many of them. Arms like pipe cleaners.