Approximately 4 miles. Clockwise description: Start at Romney Gym on MSU campus (4910' elevation). Slight downhill sidewalk east on Grant St., cross Willson, and join Gallagator Trail. Head northeast (left) on Gallagator, slight downhill gravel to Burke Park (4820' elev.) Follow Boyd Trail (gravel) for continuous uphill (about 1 mile) to top of Peets Hill (5060' elev.), then continue south downhill on dirt/gravel to Kagy Blvd. Turn west (right) and continue downhill on sidewalk for 0.5 miles along Kagy, then gradual uphill to S. 7th. Flat return on sidewalk to Romney Gym.
There are a bunch of additional loops that connect to this one for longer runs (Highland Ridge Trail, Sourdough Trail, etc.). See www.gvlt.org (Gallatin Valley Land Trust) for more trail info in Bozeman. Also see http://www.coe.montana.edu/ee/rmaher/pers/running.htm for a few more of my favorites near the MSU campus.