A great adventure from the Trail of the Hiawatha near Taft, MT all the way down to St. Regis, MT. The trail is complete with a tunnel and several tressels and loads of viewpoints along the St. Regis River.
The trail parallels the river most of the way and even dives under I-90 a couple of times. There are a few places to access the trail by car all along the way so don't be too worried about the length being 32 miles. You can easily run it in sections.