@gravity3 I actually suffer from low testosterone and rub testosterone gel on my shoulders after a shower. Ironically my son who is 43 also suffers from low testosterone.
If you want a great exercise try doing planks. That and leg lifts are great for your core.
Keep it up. The older we get the more we have to push ourselves. I used to do the stair master but I found it boring. I use a machine similar to an elliptical but harder. It works my arms as well as my legs.
Good luck.