Monk's walk

Here is an example of a velocity puzzle.

Puzzle

Famed TV detective Monk leaves his house for a leisurely walk. He heads due north, walking 3 miles per hour (touching all the parking meters along the way). Suddenly, he remembers that he might or might not have left the oven on, so he books it back to his apartment, traveling twice as fast. The oven was fine, of course, but Monk noticed that he had been gone for exactly 15 minutes. How far did he walk?

References

I was inspired for this puzzle by The Riddle of Scheherazade: And Other Amazing Puzzles by Raymond Smullyan..