Can I put fitness/gym as an activity on the Common App?

I’m a rising senior who gyms at least 9 hours a week regardless of where I am or how busy I may be. I was a JV athlete in 9th and 10th grade but dropped it after I realised participating in sports was harming my body. I basically kept getting injured and/or sick. In the summer of 2015 I started working out. I dropped ~30 lbs and have maintained my physique ever since. I’m enjoying it much more than being an athlete. I’m healthier and happier. In fact, ever since I started working out my grades have improved drastically as well. I’m tackling a tougher course load with way better grades.

Should I include it as an activity on the Common App? It’s definitely quite time consuming

Absolutely.

putting that on your application cant hurt you. If anything it would help

Just to be clear, it’s not like I’m short on activities to include or anything.

It might eve be a good essay topic because it seems to have impacted your life /your choices in a major positive way.