I'm still a newbie too, but I may be able to help since noone else has answered. A character will walk for however many frames you tell them to walk. If you want the character to stop after the walk has already been applied, just delete all keyframes after the frame you want the walker to stop. What I did was create a 30 frame movie, had the walker move for 30 frames, then added another 200 frames for the rest of the action. Dunno if its the best way, but it worked for me.