Suicide lol
On a serious note, many people live an unhealthy lifestyle for decades, and they're ok... I know of several. Don't presume it'll kill you quicker, there is absolutely no guarantee. I assume it wouldn't be a nice death anyway, probably slow and painful.
I have a friend that is very large, and she has been for decades, and she's only in her early 30s. She doesn't exercise, as she believes you can be healthy at 'any' size, which is crap.
When she walks up a flight of stairs, well, she's fucked, she wheezes and all sorts. She quit smoking a few years back. When she's merely sitting, she's out of breath... I'm a smoker, with asthma, a healthy weight, and I work out a lot, and I do other things as well as my intense workouts 3-4 days a week to keep active.
I'm pretty active, and working on constantly getting fitter as it is good for the mind, it is impossible to be too fit, unlike being unfit. I can easily run up a flight of stairs without dying. I couldn't imagine anything worse.
I'm working on quitting smoking (again), but I'm as fit as a fiddle. I remember when I wasn't - it was awful. So embarrassing, I felt so gross all the time. Trust me, you DON'T want this. Sedentary doesn't mean you'll be that big, of course, but it will make you THAT unfit. You'll feel so shit. You can be slim and ultra unfit.
Why do you want to do that to yourself? I don't understand.