I take the most direct route in (6.5mi) so I can lift or do abs at the gym before I shower, and then take varying routes home, just to mix it up a bit. Sometimes I go a mile or two past home to the store/restaurant/library, some days I add only maybe 1/2 mile but it contains a downhill then a steep uphill I wouldn't normally hit, etc.

My commute is going from 6.5mi one way to close to 12 when I move in 2 weeks, so I'm sure for a while I won't have the time or energy to be lentghtening it, but I will be experimenting with different routes.