I am or was an avid bicyclist, on a road bike alone I could maintain 25 by GPS or Speedo for short distances. In a group an amateur can do this and better for a while under good conditions. Professionals of course can do it longer. Kitted up on a trials bike they can do better. A bear would likely still catch them unless they had a start.
Wind resistance increases as the square of the speed so without drafting you hit a wall.
As far as running goes, the bear can do better almost 400 feet in 9 seconds which is better than most sprinters, and while he isn't a distance runner he is a good quarter miler.
On a MB, with MB gearing, and soft trail, very few are going to hit 20 much less maintain it for any distance.