hey i'm 20, i've been on hrt for the better part of three years, so my experience, physiologically at least, shouldn't be too different.
here are some things about hrt:
— it takes time. it took me six months before i saw appreciable changes. it may take less time, it may take more time, because the thing about human bodies is that they are all different!
— your epiphyses have probably fused, which means that most of the skeletal changes to your body have completed (most of this happens by age 18 or 19), but again, everyone is different, and your particular bones may still be growing. there are still minor changes that occur after epiphyseal fusing, which may or may not be appreciable from person to person.
— there are certain changes associated with the decreased muscle tone and oxygen-carrying capacity of an estrogen-predominated body. however, some of the things that have been found to decrease mechanical output (especially in regards to running) in women have to do with musculoskeletal differences. if you are mostly or wholly musculoskeletally mature, you may actually find that your running doesn't suffer dramatically. you know how they talk about "runner's hips" in women? male-assigned people usually-but-not-always have that by default.
transitioning in college (even though it was something i'd mostly finished by the time i arrived) is...weird and unpleasant, from the little i experienced, but i imagine it is MUCH better than doing so out of school and in a job. it really is the sooner the better, so, pragmatically speaking, doing it now is the best time in your life to do it.
