I've been recovering from a knee injury for the past few months that I got playing football (soccer). Initially, when my knee was too sore to run properly, I started using the elliptical machine to stay fit. Gradually increasing the resistance, after a few months I was running on the highest resistance setting for 30 mins, and had built up quite a bit of muscle on my legs. My knee was feeling a lot better, so I started playing football again. When I play I feel fine for the first 20-30 mins, but after that my knees (and legs in general) feel weak, and tire easily. When I run on the elliptical machine on full resistance it's hard work, but is it having a negative impact on me playing football? Would I be better off running on the road? I understand that elliptical machines are easier in the joints, are my knees unused to strenuous activity and so when I run on a harder surface they become strained?
Although elliptical machine can simulate running, it is very different than actual running. It's really good to recover from an injury and a great way to get the legs back in shape but to be able to run, you have to start running. In my opinion, you can try the couch-potato to 5K run since it's easy and builds your running abilities quite fast. And everything should be done while being in supervision of an appropriate medical treatment.