I thknk that Old Man Willow, is awakening (or has awakened), but went through more of a mental than physical change. He can move his brances, but they're still branches, not arms as such.
Anyway, I get a sense that because he's so old, his roots have spread throughout most of the forest. Like they've sort of become mixed with the ground, and are entwined (not entwifed
) with all the other roots of younger trees. It sounds like that to me when treebeard describes it.
I don't think he really needs to walk.