First concern it to verify that you're healthy. Otherwise you could set yourself up for a massive heart attack. I have a friend that thought he was healthy but went in for a stress test. He literally died on the treadmill and was only resuscitated because the doc & staff were there to catch him.

Second up after getting the OK is to understand that sweating is good. Improving means pushing yourself to the point of suffering. Thus the remarks about pushing through pain. The body gets stronger as a response to stress. If you stay in your comfort zone, don't expect much improvement.

Third, REST. The body's response to stress only happens when you're resting, so rest as hard as you work out.