Today the goal is to do nothing more than an easy walk and a three-minute initial assessment followed by an easy stretch series to help you mentally prepare for the month of exercise you have before you.
Outside or inside, it's your call. Or the weather's call. Take a 10- to 20-minute walk just to get your blood flowing. You don't even have to push yourself -- the point here is to start developing a habit and get yourself a little warmed up before performing your initial assessment.
Do 10 minutes if you're feeling especially crappy, 20 minutes if you have a moderate amount of energy.
The initial assessment determines a baseline score for lower-body, upper-body, and core strength that you can compare yourself to at the end of the challenge. All you need is a timer and somewhere you can record your scores (make a note in an app, or use old-school pen and paper).
For the first two exercises, squats and push-ups, you want to record how many you can do in 60 seconds. For the final exercise, plank, you want to see how long you can hold a plank. As soon as your form starts to suffer -- your back sways, you cheat by pushing your hips to the sky, or you sag your chest and head between your shoulders -- the time stops.
Remember to keep track of these numbers, they're important for measuring how far you've come from the soft, hungover person you used to be!