I’ve been struggling with obesity throughout my whole life (BMI of 46 at my high point which is damn high) and never been particularly fit, strong, or into sports.
I don’t have a car (though I could afford one) and go nearly everywhere by bike. Non-electric bike. My hometown (Dresden, Germany) is relatively – but not totally – flat in all the places you might want to get to. That certainly helps. (Though the nearly non-existent biking infrastructure – except for painted lines – certainly doesn’t. But Dresden is not terminally car-centric, so it’s alright to drive there by bike.)
I mean, just for moving around the city (usually around ten kilometer roundtrips, maybe the odd twenty kilometer roundtrip on the odd day where a more distant destination is where I want to get to) that just works for me without any problems, no matter how heavy I was. That doesn’t mean I go places at 30 km/h, in fact I might only go 15 km/h – but that’s still several times faster than walking and actually pretty competitive with cars and public transport.
And while walking got harder as I got heavier (mainly just in terms of causing pain after relatively short distances), while climbing stairs got much harder as I got heavier, biking (and swimming!) honestly never felt that way to me, at least not for normal roundtrip distances around ten kilometers or so.
I use bikes to get to places, to move throughout the world, not for sports. And for that you don’t need to that fit. And with an e-bike? That required fitness drops even lower. (Unfit seniors getting stuck somewhere in the alps with their rented e-bikes is a real classic …)