Ben, if I may make a suggestion. Stamina and skeleton (knees) will both be helped if you totally give up the tobacco. (I assume that's what I see in your avatar.) Smoking drives up the amount of carbon monoxide in your blood. That binds more or less permanently to hemoglobin in the red blood cells, rendering those blood cells incapable of carrying oxygen to the tissues. Bone is living tissue and has to have oxygen; but it's not as well vascularized as other tissues in the body. Blood with incapacitated RBC's can't deliver as much oxygen to your tissues. So cutting out the tobacco will make a difference in your stamina and post exercise recovery as well.