Finally, FINALLY, I felt that my knee was stable enough to risk the treadmill at my school gym. With my gym buddy there to catch me in case it all went wrong, I did five mins warm up walk, then upped the pace and went for it. I did 3.17 miles in total, and the knee, apart from one slight twinge as I upped the pace, was very good, no pain. I did a pile of stretches, and some mat work, (crunches, etc) afterwards and just generally loosened everything up a bit. Felt so good to have those endorphins racing around my body again.
I'll continue with the quad/hamstring strengthening exercises and try to get out of the door on Wednesday sometime for another 5k'er. I know I need to take it easy and not go mad, or I'll set myself back again.