Take care of yourself until you get to about 35 then start eating junk food until you get diabetes. Basically you want to get your blood sugar to be around 350 most of the day most of the time. This will slowly cause organ damage. At some point you'll have to go to the doctor and get meds so you don't die too fast, but if you play your cards right you'll develop nerve damage and get to experience a reasonable amount of pain pretty much all the time.
If you can try and hurt your back sometime before turning 30, that'll really help make things worse. You know how they always say "lift with your knees?" Yeah, just don't do that ever and go out of your way to lift heavy shit badly. Also, be sure to get married to someone really annoying before you start your diabetes run. Maybe become a Mormon or join some other religion that frowns on divorce and become SUPER ACTIVE in the church. You'll not only suffer because that environment sucks you'll have a spouse you don't like who can contribute to making you miserable. Kids are optional, but having several shitty kids is a very quick way to make yourself miserable, especially if you can't afford them.
You want to get married before becoming diabetic so that your annoying wife (and potentially annoying kids) will have something to bitch at you about. Be one of those freaks that hides Little Debbie snacks in random places all over their house, sneak out in the middle of the night to go to 24 hour fast food places. Basically give lip service to getting better while deliberately sabotaging any progress. If you start actually losing weight (somehow) increase your carb intake.
Finally, after hitting age 50 troon the fuck out. Go all in on that shit, get the hormones and sign up for the fake pussy. Go ahead and lose some weight so that your organs won't stop working altogether, but not enough to be considered fit. By this point your wife might have left you. Either way at this point commit to becoming an alcoholic. Stop eating junk food but replace that shit with alcohol, preferably liquor. Be the troon at the bar constantly trying to flirt with anyone and everyone, become a penis receptacle and, if still married, start bringing people home to "share" with your wife.
With a little luck you can catch some interesting diseases and finish up the back half of your life an alcoholic troon with AIDS, peripheral nephropathy, a fatty liver that is also scarred from alcohol induced cirrhosis, possibly have other organs give out on you from the badly treated diabetes, maybe even go blind. From here it's basically just waiting to see what kills you and you aren't really going to get any better.