Vary your planks based on your fitness level and core strength. Vary from traditional planks on elbows with core engaged and back flat along with side planks either on elbow or hand (key is to engage your obliques and not dip into shoulder). You can also extend top arm and/or leg to make more difficult. Variations of regular planks include pulsing one leg up and down, alternate raising one arm and one leg, elbows on a stability ball, feet on a med ball, be creative (again if you are able). The goal with this phase is to hold as long as you can, rest for 15-60 seconds between your planks. I suggest stretching rather than just resting.