@PurpleLilac, gently, get that treadmill back out - or sell it and buy another one, different color so the trigger is not so great that it's the same treadmill. Used sports stores have working treadmills for really reasonable prices, and they'll take your old one, too.
Don't let the trigger make you unhealthy.
Read my post above, we're in the same boat - my WW is really into fitness now - she was always on and off with it but really got into fitness again around the time she met OM, and he's the outdoorsy, construction worker, perfect health and fitness type, while I'm very much an indoors type (naturally stable "normal" weight for my height, but not in shape at all). She looks great now. But I'd rather have my slightly chubby (she'd say very because of image issues, it wasn't true) honest wife than a WW who looks great in yoga pants but will run off and fuck another man any day of the week and twice on Sundays.
Having said that, as we are processing all this we have a lot of anger and issues, and working out and getting moving really is something we could be doing with all this pent-up energy instead of letting it turn us bitter. So yes, it feels like work, yes, it's a trigger, but I do it, and carefully allow the mind imagery and anger to surface a little bit while working on the iron machines. I push harder than I ever did, and I just started a bit ago but I expect I'm going to see some real results soon. If nothing else, I feel a lot better when I get out of there.
She can eat her heart out when strong, idiot OM dumps her for good and stronger, better looking ex-husband tells her "sorry hun, you had your chance to recover, I'm going out with some singles tonight again with the body you helped create with your betrayal, see ya". Childish? Yes. But it's a fantasy of the future I have. And if it doesn't happen, I'm still healthy, for myself, my kids and whoever comes next in my life. I owe this to future GotPlayed.
Don't let the trigger destroy you. It wasn't your fault, but you created it in your mind, which means you can sure as heck destroy it.