From my own experience, running around 4 miles a few times a week has made me feel more energetic and healthy. While I'm not an expert, I did chat with a running coach San Francisco who mentioned that moderate running, like your 12-20 miles per week, generally shouldn’t negatively affect T levels. In fact, they said that this amount of running can help improve overall health, which might support hormone balance. The key is not overdoing it and making sure you’re also doing some strength training to complement your running.