Every good runner knows she should take off at least one day a week, but some experts— including Bill Pierce, director of the Furman Institute of Running and Scientific Training (FIRST) in Greenville, SC—think that number should be closer to four. Pierce developed a program called “3 plus 2”: three runs each week with two cross-training sessions, along with some strength training and stretching. The runs include a track repeat workout (building aerobic capacity), tempo (anaerobic threshold training), and long run (to gain endurance). Cross-training workouts include non-weight bearing sports like cycling, swimming, or rowing. “The goal for these is to build aerobic fitness without impact, which can lead to Achilles tendonitis, stress fractures, knee pain, and hip pain,” Pierce says. Running muscles can recover though you’re still working hard during the cross-training sessions, doing away with easy days and increasing the quality of your workouts throughout the week.