Workout length poised for a regression to pre-graduate school levels

It appears that 2004-2006, I averaged about 50 minutes per session, while in 2007 and 2008 I averaged about 20 minutes more. The most obvious driving force here is that I attended graduate school September 2006-June 2008, which left me with much more free time than when I was working.

So, what caused the longer sessions? Was it simply that I didn't have work waiting for me at home, and that allowed me to run farther? Partially, yes. I began playing squash on weekdays, something that had been strictly a weekend pursuit for me in the past. That sort of gain is likely to be reversed. Then again, having more time off also inspired me to set larger goals. I ran a marathon in the spring of 2007, and started lifting weights in spring of 2008, both of which made me spend more time at the gym. Those gains may be maintained.

A shift in the type of exercise might also help explain the change in workout length, but the exact composition and its change is unclear. It's also unclear why 2006 shows a decrease in workout length when the last 6 months were in graduate school or on vacation.

It seems clear that when predicting the future, the rise in number of workouts could be sustainable, but that the increase in workout length may or may not be in for a regression to the historical mean, depending on whether changes in exercise mix and goals or weekday squash was a larger driver of the 2007-2008 increase.

Of course, there is a significant observer/actor bias, but we'll assume that exercise is driven by stronger forces than graph creation.