I started monitoring and graphing my weight when I started on HT, so I could see what was happening and try and avoid gaining weight and losing muscle. I bought body composition scales so I could see how much of my weight is fat, muscle, other, and proportion of fat which is visceral.
I'm a cyclist, and I've kept that up. My onco is a cyclist as well and we've done a couple of the same events, and even came joint 3rd in one event, just 7 weeks after he'd stuck 18 brachytherapy needles in my nether regions. I think cycling exercise might have reduced some of the side effects from the treatments. (The cold wet weather has kept me in for the last month though.)
I've managed not to lose any muscle on HT by exercising, but my fat is increasing, including visceral fat - it's gone from 9kg to 13kg in last 10 months.