I injured my knee playing football with the dubs - lol - a few years ago. I was advised to do isolation exercises for my knee once it had healed. Front and back, quadrecep and hamstring exercises. I was back to running very quickly.
Aim to walk and run - 2mins running 2mins walking to build endurance during your training. If your knee starts to hurt during the running segments then walk more. You'll gradually build stamina.
good luck. I've often though about doing a half marathon but never signed up. Have done 10k in 54 minutes. My moment of glory