Assuming most of the Garmins have the same general menu system... On my 405 you'd select the following: Menu, training, options, autolap, by distance, then toggle up or down till the distance is set at 1.00 miles. Once you've done that, when you upload to Fetch or Runningfree your mile splits will be easy to check out.
Edited: 03/04/2012 at 19:41