A steady run is a good way to stay in shape, but running intervals will really push your cardiovascular fitness to the next level. Whether it’s sprints or hills, all you need is 10 seconds at a time. “I recommend keeping the intensity at 10 seconds to ensure you go all out,” says Fox, who suggests starting with four 10-second intervals per workout, eventually working your way up to 10. He also recommends doing the intervals first, so not only do you have the energy to do them, but this type of training will deplete some of the glycogen or carbohydrate stored in the muscle, allowing you to tap into stored fat more readily. Bonus! Work hard enough to get out of breath and then take whatever minimal rest period you need to recover, says Fox.