Because the vigorous resistance-training is supposed to start well before you start the meds. That way you’re working to maintain the lean muscle mass before the hormone therapy wipes you out.

Here’s a video from a recent PCRI conference about hormone therapy and the physical benefits of exercise:
> https://m.youtube.com/watch?v=YE61HSAsFb0

Pushing through might not work, because it could knock you right into bed before you see any benefits. It's more a matter of bootstrapping: make a lot of little choices every day, instead of sweating it out forcing yourself to do hour-long exercise sessions. Eventually, they start to pay off, and you have a little more energy that you can use to get a little more active, then being a little more active gives you a little more energy, etc. in a virtuous circle.

Some examples of little choices:

1. Park your car a little further from the store.
2. Get off the bus or subway one stop earlier.
3. Walk to the corner store to get milk.
4. Curl some very light dumbbells while you're watching TV.
5. Stand up from a chair sometimes without using your arms to push.

Etc. Even with baby steps like that, you'll have setbacks, but eventually (I hope) you'll see a positive improvement over time. Working with a physiotherapist, rather than taking advice from random online strangers like me, would also be a good idea. 😉