I've seen hippos on dry land twice at full speed.

The first at about 15 metres in a swamp in Kenya at a waterhole. We only had cameras but it was greased lightning running across our front to hit the water. Being a tourist group I wonder what would happen if the hippo decided to go through the camera jockeys?

The second was on a boat on the Zambezi. Someone had disturbed a hippo feeding higher on the hill in some bush and it came down at speed straight for an African fisherman at water's edge. We shouted for him to watch out, and luckily it veered. That hippo coming down a fairly steep bank would have been very hard to stop by shooting if on the ground. It is amazing how fast they can run too.