Solve the following math problem: Jonathan enjoys walking and running for exercise, and he has three different exercise routines.  On Mondays, he walks at 2 miles per hour.  On Wednesdays, he walks at 3 miles per hour.  And on Fridays, he runs at 6 miles per hour.  On each exercise day, he travels 6 miles.  What is the combined total time, in hours, he spends exercising in a week?