I like running too and have been wearing a sports bra for the last two years.
Initially because running is something I like to do for me and have been doing it for over 30 years. I would put some extra breast (filling) in my bra to feel how it was to run like a woman with breasts, and I that gave me a great feeling and gave me an idea for the maximum size I would like to have. I tried with too much while trying to figure out my size and it really gets uncomfortable.
I am currently on HRT, the breasts are appearing and I keep on using the bra.
It does really make it more comfy and, as I run early in the morning, it prevents the nipples from chaffing.