I just wanted to say one thing. Don't let your mind convince you that you are too tired or that you can't do it. Not just for this exercise but for any, when you feel like you can't do another, push yourself, tell yourself you are going to do one more and do it. Our bodies are capable of more than we think and just because it's hard doesn't mean you can't push a little more. A lot of the time we stop after we think we can't keep going. You will only get better, stronger and the results you want if you push your body and challenge yourself both physically and mentally.