When you're getting tired, it's usually your mind that gets you to stop, not your body. Your body CAN do more, but it will be difficult. The next time you start to feel like you can't go any farther, just think, "I will do at least one more." If that's one more rep, one more block or one more step, you can do it. After you've done that one more, go back and think, "I will do at least one more." Keep repeating that until it becomes physically impossible to do one more. You will impress yourself with how much more you can do when you put your mind to it, instead of against it.