I've been fitter and I've been way unfit (in my 20s I was a gym rat), early thirties I ballooned to 245lbs and these days I sit around 185lbs (because of health reasons I struggle to eat enough to keep weight on) having lost 60lbs a few years ago via diet and exercise.
For someone approaching 40 I look decent physically (which is actually a problem since I have multiple serious health problems).
For me the cycling is important, it's both a fuck you to illness and a pressure valve - as a friend said once "it's impossible to be remain angry on a bike" as you just exert yourself harder.
Added to that is calisthenics every other day (planks, pressups, sit-ups and pull-ups) as I'm focussed on maintaining functional strength as much as I can.
But true your milage may vary, but usually it is just discipline issue. People want instant gratification and do not want to wake up early to hit the gym before they head out to work.
Finally, I learned to get on a good program and TRACK MY MACROS. That's making a huge difference.
It's not always discipline. Sometimes it's finding the right program. It took me literally 20 years to figure that out, because I was laboring under the bad advice that "you should just eat healthy and you'll get there" that my dad kept telling me.
For the naturally skinny, you need a constant caloric surplus (or even just a regular amount of calories to recomp -- I couldn't even recomp effectively because I'd starve myself) or you're mostly wasting your time.
That's a sizeable investment when you have kids. Which is fine -- but you definitely have to stop doing something else in order to do it. If like me (and perhaps parent comment) you _already_ don't passively consume media, go to bed early, etc, you really do have to trade something for it. And (again, perhaps like OP) if you are already healthy and fit (ex: I can play soccer for 2 hours) there's even less benefit (relative to your other pursuits). Which is all actually to say -- there are many people for whom discipline is certainly _not_ the issue.
All my meals are already home-cooked (and, in the summer, mostly home-grown), and I exercise for 30 mins 5 days a week (plus extra physical work in the garden at weekends). I'm fit enough. The extra couple of hours I'd need a week to build muscle are better spent doing something else, IMO
Judging this is real hard since fairly fit means such different things for different people, but spending 5 hours a week at the gym isn't a big time investment. It obviously won't be getting you Mr. Olympia body, but it will make you bigger and stronger than most people.
Part of discipline is also fighting against excuses. Excuses like I don't have time to work out. I don't have time to prep my meals. That's why you need discipline.
Taking things to extreme you could wake up at 4:00 am to workout for an hour every single day. Obviously that means giving up on media consumption time during evening to get full night's of rest, but that is why you need discipline.