If you can get over to West Yellowstone for a day, run on the Rendevous xc ski trails.
http://www.westyellowstonetraveler.com/features/rendez.shtml
...really, really nice. I ran on them last summer on vacation. Soft surface, winding, lots of small hills, pine trees. Kind of what I've always imagined running in Finland or Norway would be like.
If you make a normal amount of racket, you're not likely to see any black bears. Bells may make you feel stupid, but the idea behind them makes sense. Black bears are not into seeking out encounters with people. (Unless they've been fed by people.) Bison are kind of spooky though. They fairly regularly gore and trample tourists who get too close. They don't get out of your way when you approach, like most wildlife will. They just stand there and watch you. If you encounter them on a trail (usually in open areas) I would give them a very wide berth, or else turn around.
Check out this bison getting pissed and flipping this dude into a tree:
http://www.nps.gov/yell/kidstuff/testknow/images/bisongor.htm