I don't think mercs can run. At least, I don't recall seeing two different movement animations (one for walking and one for running). Maybe an MPQ-diver could say for certain whether they even have a running state.
They do indeed have a running animation. For the Act II merc, you can see him walk around in town and he holds the spear with both hands. Go to a WP and he will drop the spear to one hand to run towards an enemy.