If you're on T, you probably won't gain much fat in the "wrong" places.
It's likely muscle gain, especially if your clothes still fit but are just tighter.
It could be a really good experience for you to get used to the higher number for your weight. Being right at a healthy weight instead of under or over can feel great, but can also be psychologically weird. Still, once you get past that weirdness, it can feel psychologically good too.
As active as you are, it's unlikely that you'll gain any weight from fat, and if you do more cardio than resistance/weight training, you probably won't gain as much muscle as if your exercise was the other way around.
No idea how much longer you'll keep building muscle if you don't change your routine. That's probably different for everyone.
Be healthy, enjoy life, feel awesome. Congrats on being healthy, being on testosterone, and joining Susan's. All good things.