Until the power grid shuts off and you have to manually push the treadmill up to speed and maintain the speed?
Just go run up some stairs...
Look at people who live on farms, or don't get their food from grocery stores in general.
They are fit without going to gyms, whether it be from working the field, having to get water, or hunting and carrying the kill back, etc.
50 years ago people were a lot more fit just by not having trash food to eat at every corner, not because they all hit the gym every day.
The point is that it is now common knowledge that weight training is by far the most effective way to build and maintain strength, which you will most likely need during an apocalypse. The strong survive.
here's a question … do you work out knowing your the last person alive???I'm sure it's good for health and you'll survive longer but do you really care what you look like , are you gonna be working out your glamour muscles and staring in the mirror. just surviving in that environment will have your six pack abs cut in no time ...treadmill... ha it's called running from giant super mutants
But who will wash my sweaty towels?
Just go to the next gym when you run out