I have run three marathons, my best was 3:32 in 2012, so I have run a lot. Unfortunately, I had let myself fall out of shape during the year before I started HRT. Since then I have started running again.
The biggest thing that happened to me was a decline in my red blood cell count. This may have been partly due to four blood donations since last July. I am now trying to recover that.
Beyond that though my running fitness no longer improves as rapidly as before and my average easy run pace, measured by heart rate, has declined by 1 1/2 minutes. I also feel unable to train as hard as I used to although that might just be a mental thing since running used to be a primary coping mechanism for dysphoria. I used to pretty much flog my body six days a week and still improve. I am running six days a week again but not punishing myself nearly as much as before.
A couple of years ago I could push out a 6 min mile (max effort) after several months of training. Now that's also around 1 1/2 min slower.
Complicating that too is my age. Maybe some of all that is age related decline.
Form wise I have not noticed any change. I run minimalist in sandals so that may have something to do with form consistency.
One good thing is that maintaining a lower body weight for running is easier than it used to be. This seems to contradict the experience of others on HRT. I do pay pretty close attention to my daily calorie balance to get that. For a few months last year I quit paying attention to it and started gaining two pounds a week. Left to my own devices I can eat a lot non stop, LOL. Fortunately, at the start of that I was pretty skinny so even with a 21 pound gain I was still in normal BMI and bodyfat ranges (my avatar pic was taken around that high point). Half of that gain is gone again since the beginning of the year leaving me in a pretty good place right now.
Beyond all that though I still enjoy it a lot, nothing hurts, and it still feels just as good as before.
It is better to be hated for what you are than to be loved for what you are not.
André Gide, Autumn Leaves