If all you were doing was running, it would be fairly manageable. Unfortunately for the average distance runner, you will need to be able to do all of the mileage wearing a full pack or while carrying an alternate load or while alternating heavy resistance movements and planning/executing mission objectives. (I hate to say it but a high level cross fitter might be better prepared for this type of training than a distance runner).
During hell week, rest (including most sleep) is eliminated or greatly reduced. Operating effectively when every cell in your body is screaming to stop and/or sleep is not normal.
Most, of not all, of my SEAL (and Ranger) friends have told me that the physical training is challenging no matter your athletic background but the mental strength component it what truly separates those that make it and those that wash out.