I'm on my phone, so hope to type more later, but my "old" dog (just turned 6) has been thru hell the last few years. Cancer (huge surgeries, chemo, etc), hip surgeries, tplo, muscle injuries due to compensation, etc. He is truly a dog that NEEDS to work, it's really the only thing that makes him come alive, have a spark in his eye, etc. So (with the blessing of his surgeon and rehab vet) when he is sound, he works...and he is happy.
Right now he is managed thru supplements, chiro, massage, laser and rehab, but NO DRUGS! WOOT! I try to make him take it easy, I don't let him do the work that is truly hard on him, but I challenge him thru obedience and tracking, easy agility and field work, etc. I still hold my breath every time I watch him jump or hop, but he is HAPPY! With this boy, it's all about quality over quantity of years at this point. He's been down and recovering more than not the last couple years so he deserves to play when he can.
I've been slow to let loosen the restrictions and let him be a dog, but our rehab pro has kicked my butt telling me that it's good for us both to continue to work/train...and I know she is right!