I sometimes walk on the beach for up to eight or nine hours in one stretch.
Although I definitely have a "purpose" in being on the beach-- beach combing for rocks and other interesting objects-- these long wanderings are as much a "walking meditation" as anything else. Walking on the beach-- with no other sounds but those of waves breaking and an occasional seagull-- feels incredibly healing and cathartic.
At times, my mind goes completely silent, and all I am aware of is the sound of the water, and the feeling of my feet on the sand. Everything else vanishes. Time seems to become very fluid. Sometimes I look at my watch and 45 minutes will have passed, even though it feels like a moment. At other times, only a couple of minutes will pass, but it feels like I have been "drifting around" for hours.
On many occasions, the walks are a way for me to solve problems in my head, carrying on dialogues with myself about how to address some issue, or develop some idea I've had in mind, for a while. When I come off the beach at the end of the day, I typically have much more clarity than when I started.
Of course, my memory isn't what it once was, so I always carry scraps of paper and a pencil so I can jot down a few words, should a moment of brilliant insight strike me!
It may not be "meditation" in accordance with certain classic definitions, but to me, it certainly is.