I’m lazy and generally don’t do off ice training. I used to do long distance running, 10km, which meant I had very good endurance and stamina, but I gave up due to dicky knees. Running or swimming or cycling would be good to get stamina. I advise against running as it puts stress on joints. My knees will never be the same again. When I get on ice, I do some drills such as inside and outside edges, and tight turns, which help me stretch. Our hockey team was taught some on ice stretches, which are simple enough. Thus sit on the ice, back straight, turn your shoulders to one side and place hands on ice, repeat on other side. Lie in your back bring one leg up then move it towards the other side, knee bent, and push it towards the ice. And so on.