The nice thing about hacking instrumental goals into terminal goals is that while they’re still instrumental you can easily change them.
In your case: You have the TG of becoming fit (BF), and you previously decided on the IG of going to the gym (GG). You’re asking about how to turn GG into a TG, which seems hard.
But notice that you picked GG as an instrument towards attaining BF before thinking about Terminal Goal Hacking (TGH), which suggests it’s not optimal for attainging BF via TGH. The better strategy would be to first ask yourself if another IG would work better for the purpose. For example, you might want to try lots of different sports, especially those that you instinctively find cool, or, if you’re lucky, that you’re good at, which means that you might actually adopt them as TGs more-or-less without trying.
(This is what happened to me, although in my case it was accidental. I tried bouldering and it stuck, even though no other sport I’ve tried in the previous 30 years did.)
Part of the trick is to find sports (or other I/TG candidates) that are convenient (close to work or home, not requiring more participants than you have easy access to) and fun to the point that when you get tired you force yourself to continue because you want to play some more, not because of how buff you want to get. In the sport case try everything, including variations, not just what’s popular or well known, you might be surprised.
(In my case, I don’t much like climbing tall walls—I get tired, bored and frustrated and want to give up when they’re too hard. One might expect that bouldering would be the same (it’s basically the same thing except with much shorter but harder walls), but the effect in my case was completely different: if a problem is too hard I get more motivated to figure out how climb it. The point is not to try bouldering, but to try variations of sports. E.g., don’t just try tennis and give up; try doubles and singles, try squash, try ping-pong, try real tennis, try badminton, one of those might work.)
How about extending the metaphor and calling these techniques “Rituals” (they require a sacrifice, and even though it’s not as “permanent” as in HPMOR, it’s usually dangerous), reserving “Dark” for the arguably-immoral stuff?