Chris, ask for some help from physios, and dieticians right at the start (it's much easier not to put it on than it is to lose it), and you can probably negotiate consultations with both of those through your onco or CNS as part of starting HT, or even beforehand.
I don't know what constraints you have on your knees, but would cycling, rowing machine, swimming (all no jolts) be possible? I do a home exercise class 3 times a week on Zoom which is for cancer patients (we're nearly all prostate cancer), and the trainer takes into account each person's limitations, and gives them something different to do when necessary - she would discuss this with you beforehand. I do them pretty energetically, and consume 500-600 kCals each class, which is baseline of over 1500 kCals/week before I even get out on my bicycle (which doesn't happen in cold wet weather). But it's at your own pace - most people are older than me and take it more gently. The only things needed are hand weights (I'm still using two Pepsi bottles) and an elastic band (which the trainer sends you). There's a small charge (we made her charge, to keep it going during COVID).
The other thing is how to protect bone health which is normally done by stressing bones, but that needs guidance around where any bone mets are, so again, ask for physio support to undertake this. They might decide to do it just with drugs (such as calcium, Vitamin D, bisphonphonates/denosumab, etc.), or that you don't need anything.
You've probably seen me mention this before, but I also bought body composition scales so I can monitor my weight and composition of muscle/fat, which I do daily, and try to keep an eye on it. When I'm over my self imposed limit, I skip breakfast. It's also interesting that I found I need quite a lot of protein in my diet in order to maintain my own muscle mass. Without the body composition scales, I would not have noticed I was losing weight by losing muscle when cycling a lot, and eating more protein while exercising reduced the muscle loss.