Fatigue is a common symptom of PCS. The neural pathways in your brain have been disrupted, so things that ordinarily took very little energy now take a lot as your brain has to develop new pathways for things. You can't operate on "autopilot" any more, basically.
This is all completely normal.

I agree that you are overdoing. You need to lower your expectations for yourself A LOT. You shouldn't be anywhere near the stair climber. 20 minutes of WALKING on the treadmill would be more like it. Something you can do at a speed that will not make any of your symptoms worse. If you feel something starting to get worse, if you feel that you are having to "push through," STOP IT. Or go for maybe five minutes more, but no more.

Nine holes of golf is huge. That is a tremendous amount of walking. There is a lot of sensory input. You are making lots of decisions about which clubs to use. No wonder you are exhausted! If you want to play golf, maybe try a driving range instead or use a golf cart.

I completely empathize. I was living my life at 100 miles per hour before my concussion. After the concussion, recovery was at about 20 MPH. My new normal, 2 years post-concussion, is still only about 50 MPH. It's a slower, saner speed. I don't know that I'll ever go back to full tilt like I was before. I miss the old me, but there are things to appreciate about slowing down, too.