The automobile is why modernity is out of shape. The most simple solution for most folks is to live in an area where, instead of sitting at a de facto lounge chair to move from place to place, it's done by walking or cycling.
This may not solve issues of being over-weight, which is mostly an issue of diet and modern food culture, but it will increase general fitness.
The "simple" part could be up for debate, but in what way is "replace some of your car/bus/train travel with walking or cycling" not a solution to the problem of staying in shape when you hate going to the gym?
It may not be feasible in all cases, but it's definitely feasible in some cases, and it absolutely works.
In particular, bike commuting is an excellent way to integrate exercise into your daily routine without going to the gym or the tedious exercise-for-it's-own-sake activity. It's not practical for everyone, but between power-assisted e-bikes and the observation that you don't need to do it _every_ day, it's a lot more achievable than many think.
Similarly, if you take public transport to work (or elsewhere) you could get off a stop or two early and walk the rest of the way to get some of the benefits even if lacking the time or stamina to do the whole trip on foot.
Integrating exercise into some task or activity you were going to do anyway (e.g., commuting or running nearby errands, if applicable) is much easier than trying to maintain discipline and motivation to exercise as a dedicated, stand-alone activity.
I am not going to walk 15 miles to Costco and carry a week of family groceries on my back. I use literally zero public transit. This advice is insane for people who live in a suburb.
A "buy in bulk" warehouse store is of course one of the least reasonable examples you could have selected, but per the Bureau of Transportation Statistics 52% of car trips in the US are less than 3 miles, and 28% are less than 1 mile. Those are both very walk-able and bike-able distances.
Surely somewhere in that more-than-half-of-all-car-trips there are a few examples of car errands that could be easily replaced by bike, scooter or foot.
EDIT: incidentally even in the most car-centric dystopian suburb there are ways to incorporate more walking into your routine. The next time you go to a strip-mall or big box store you could park on the far side of the lot and walk to the door. A minor inconvenience in both time and effort, but you could probably add 1/2 mile or more of walking to your Saturday afternoon errands that way.
A quarter mile of dodging cars with an ungainly, loaded shopping cart sure does sound like an "inconvenience," and not one I would recommend to a friend to improve their fitness.
But I stand by the statement that most able-bodied Americans could find some way to incorporate more human-powered transport into their routine if they really wanted to. And in many cases they could do it without any adding any superfluous travel. I.e., it may be slower or even less convenient in some way, but it was a trip you were going to make anyway. You're just changing the mode of transit.
(Incidentally if it's truly too dangerous or inconvenient to walk across the Costco parking lot with a loaded shopping cart I think we're well on our way to the people from Wall-E.)
You can't win debates with such people, about biking or walking; there's always another reason why they (and all other good and upstanding Americans) can't possibly walk or bike. When they run out of their own made up reasons, it's "well what about disabled people!"
Like yeah, they maybe can't, are you disabled? No? Well then.
Not sure if you or OP are in the US or not, but American cities have zoning laws and infrastructure that are designed entirely around the car. Aside from a few exceptions, towns in the US aren't like European towns in the slightest. It really is often the case that nothing is walkable or bikeable. The "Not Just Bikes" YouTube channel goes into all this if you are interested in that sort of stuff.
This may not solve issues of being over-weight, which is mostly an issue of diet and modern food culture, but it will increase general fitness.