#1 (thinking elsewhere, especially while taking a walk) is definitely true for me, but it does require being in a setting where you're allowed to leave the office for a few hours in the middle of the day to go for a walk in a nearby park. In academia or at your own startup, probably not a problem, but many companies aren't very accommodating of people leaving the office for hours at a time.
Lunch dragged on for a couple of hours sometimes, but I generally had a flexible enough schedule that it worked ok.