Not trying to be cocky here, but Day 19 is really easy.
200 high knees are good enough to exercise butt and core muscles but mild enough to keep you going. I am very proud of myself when I completed my 200th high knees and I believe this is very good for self-esteem.
Same for superman. I kind of like this workout because of the name. It is fun, and you can vary your pace as you like. Completing 100 supermen are not as hard as it sound, but I always have problem with the breathing. Instead of inhale while you lower your arms and legs back down, I always did it the other way round. I hope I can correct my breathing habit in the future.
Meanwhile, I just feel good about myself :)