Time devoted to exercise has been growing

Since December of 2003 I've kept an exercise log. It started as a way to track how far and how fast I ran. At first glance it looks like I've increased the amount I've worked out pretty steadily over the past five years. The graph raises a number of questions, though. Why has exercise time gone up? Has it been from longer sessions? More frequent workouts? Is it driven by free time or is it independent of free time? Are the averages even over the years or do they represent periods of intense activity separated by periods of sloth?

Clearly, this needs some more attention.