It involves multiple days per week of running, strength training, and potentially some cross training. Beyond your heart and respiratory system, as you’re running, you’re using a lot of muscles: the tibialis anterior (a.k.a. Jaryn Garner Garner and her coworker, Paula, pose with their medals. On the low end, you really should spend at least two days a week preparing for a marathon, Milton says. Plus, rather than a marathon being this potentially destructive experience for your body, training can give you a lot of benefits.