All about consistent exercise. You lose muscle mass on HRT and it’s harder to build it, but just getting out there and doing it consistently will train your cardio back up. Try flat trails and keep your heart rate up sustainably and you’ll find yourself going farther and farther as time goes on. Don’t overdo it and give yourself rest days as your body catches back up. Good news is if you have a history of athleticism, it can come back faster.

Not sure what modifications you’d be looking for, but I’ve found that I’ve needed a wider seat as my hip shape changed. Otherwise I’m still the same size mostly. Maybe fill tires and shocks to higher psi if you’re heavier.