220V breaker and replace it with a 120V breaker, then replace the 240V receptacle with a standard receptacle.
You'll have some spare wires, just tape them back. It'd take 30 minutes and you'd have next to no voltage drop on your treadmill since you'd be running it with #6 or #8 wire, I can't remember what has a 50A rating.
In either case, when this treadmill dies I'd look at a commercial grade 220V treadmill, you are already wired up for it!!