Because of injury i can currently only do 4 days a week(2 workouts, hill sprints and some running and a run on hills). I would like to add 2 cross training days. Now i know that i should do core, legs, upperbody and something aerobic like eliptical cycling swimming. and i will do legs on hard days so i can recover properly. i dont know when to do core and upperbody and what aerobic activity to do and how much/fast. normally i would do an easy run on that day so should it be an easy effort on the bike/ eliptical / swim? for how long?