i have same problem getting used to it...when i get off there's this weird effect where its hard to keep balance/moonwalking which tells me something is quite different about it from running even though people talk about it like its interchangeable. But you can run an infinitely long hill, feel what its like to maintain a very steady pace and do some calibrated progression runs. Probably better at first to just go by effort and relative numbers than comparing directly to running.
I always get injured running on a treadmill. I made a point to develop other avenues of exercise, so now I can swim, walk, ski, lift, etc..
Might be what I have to do. I may look at exercise bike, as at this point it is just trying to get back aerobic fitness.
Biking would probably help the knee. Strengthens the muskles around your nee (see what I did there? playing with silent K's? huh? )