From the bridge on the way to the airport, follow the river east. There is a rather random set of trails running along both sides of the river. Follow these about 10 miles to the Bayan Zurikh Tovchoo bridge on the far east side of town. You go under five bridges total including the bridges on both ends and the railroad bridge. There is no set route. You just have to pick and choose your way, following whatever trail you find. When the river is low you can cross it in many places. And in the winter you can ride the river in the entire route on the very thick ice.