When it comes to working out, the hardest part isn't necessarily pushing through that final treadmill sprint or that last bout of HIIT. The real battle is with committing—i.e., getting out of bed or off the couch and kicking those "I'll just go tomorrow" thoughts to the curb. Full-time model and marathon runner Alexandra Mack gets it. Zappos and Refinery29 tapped the fitness guru to find out how she overcomes those negative voices and pushes her body to reach its full potential every single day. To get inspired by her grind, just press play.