Is there a walking animation? When I spawned I was running when I moved, and holding shift didn't seem to make much difference. It may have been a little faster but it was difficult to tell. I tried hitting Caps Lock in case that had put me straight into running mode, but I was still not walking.
are you overweight? check your weight bar.
I've only just spawned and have picked up a few bits of wood but nothing much. I doubt I am overweight. However, even if I was, I would expect that to stop me running, not stop me walking. Or are you saying that I need to be overweight to walk? Do I have to carry around a large rock if I want to walk?
Having carried out another test, I seem to have two running speeds. I can run and I can sprint, but I can't see any way to walk.