Personally, I'd ditch the track idea and get your horse a grazing muzzle to wear during the day if there's a risk of obesity/+founder.
I know it works for some people but if you know your horse is going to be trying to bust through fences, electric or no, why not just go with it?
The other bonus of using the grass that's there is the "hello no hay bill, how are you today?" factor.
I've found that my two horses, on 5+ acres (divided into 3 smaller lonnng pastures) spend a lot of time walking around, grazing, walking some more, etc etc. Basically, those pie charts of wild horse activity vs normal horse activity - my horses are much closer to the wild horses in their daily lives that they are to the average horse.
My old lady is obese, not gonna sweet talk that one, but she is insulin resistant so pretty much all her calories go to becoming fat. She wears a grazing muzzle all day and she's not as fat as she could be but she's basically a typical super easy keeper - gets fat off air.
They aren't on any sort of "track" but their water is in one corner of the field, I feed them their "extras" in a different corner of the feild, and they're constantly moseying around looking for the tastiest grass. The path leading up to the water trough is rocky/sandy so there's the bonus of changing footing as well - they have fantastic hooves.
Anyway, it's great if you can get a track to work for you but there are other ways to effectively encourage movement. :)
On the side of finding a temporary second horse, look into finding a reputable rescue near you. Most rescues are desperate for good foster homes and since fostering is temporary, you could be doing a good thing and getting your horse a buddy.
That's actually what I'm doing right now - fostering a mare for my local rescue. It's worked out super well. I haven't had
to pay a dime (I have paid for a few things just to be "nice" but I haven't had to), my horse has a friend, and I feel like I'm doing a good thing for the world.