If you're looking for a place to do a tempo run in a trail environment, it doesn't get any better than this.
This trail is a wide, gently sloping bike path-like trail for much of the way though it does turn a little rocky and has some small rises at times. It's a beautifully shaded trail that follows Rattlesnake Creek up the valley.
Start at the Rattlesnake parking lot just outside of Missoula where there is plenty of parking and restrooms available. The trail is very wide and flat at first but will soon begin heading up though gently, less than 100 ft a mile on average. All along the way you'll find plenty of trails shooting off the main trail to explore.
The Main Rattlesnake Trail #515 ends at Franklin Bridge just over 8 miles out. You can cross the bridge and continue on Upper Corridor Rattlesnake Trail #515 which increases in difficulty.