Before HRT I was running 25 miles a week. Once I started HRT, the following things happened:
1) I lost strength almost immediately, but retained the same muscle mass so my times became a lot slower.
2) Because I lost strength, I learned that there were small things wrong with my form that I was using my strength to compensate for. As a result, different parts of my body started to hurt and I had to correct my form.
3) I lost stamina while my body got used to using estrogen for energy instead of testosterone. I had to drop down to 20 miles a week for a couple of weeks.
4) My muscles recovered a lot slower due to the lack of testosterone.
5) Due to all of these, I overtrained my body and had to take a week off.
Thankfully, all those were taken care of in less than a month and everything is normal now, with my times and distance about the same as pre-HRT. Also, my breasts started developing almost immediately, and I needed a sports bra after the first week. After the third week, it became apparent that I needed a better sports bra with much more support, since it was getting painful to run. As always with HRT, things may go very differently for you. Happy running!