You dont have to do ur cardio on diffrent days. If possible do 20mins of HITT training every morning, and keep ur weight training to the evenings.
You do need to rest, but dont make it a set thing to rest one day a week. Rest when your legs are hurting, untill they stop hurting no matter what days they are. i.e. you may run for 14days in a row and then rest for 4days, or you may run 3 days in a row and take a day off.