I disagree.
At the height of my best GMing I was in the best shape of my life (course I wasn't going to the gym). Running a couple miles a day after packing lumber up three flights of stairs, working ten and twelve hour days five days a week, building forms, framing, and tying wire cages all day/week gave me lots of time to day dream about how to handle campaigns on the weekends.
Now that I have time to work on campaigns a little I am no longer GMing, and I am in the worst shape of my life thus far. I'd love to start running again, but I can't right now, so I do leg lifts with leg weights, toss 12 lbs medicine ball from the floor to the cieling, and lots of crunches. Still no GMing. Why? Because all my daydreaming energy is spent on my daughter, trying to figure out how to repair damage to my father's relationship.
Its not about needing time to work on the campaign, its about needing time to day dream.