Yup, have done this on and off for years. When I was in college and could schedule my classes for a later start in the day, I added 30 miles to my commute. I lived about 5 miles from the college, so my ride into school was about 35 miles... but after school, I worked a part time job close to school... then rode home the easy five miles from there. Worked out quite well.
My current commute is about 11 miles either way... but when the days are long, and the weather is nice, I take an alternate way that adds about 5 fast flat miles.
Some days I explore alternate routes... although I have yet been able to find any good alternates, but none the less I explore anyway.