OK, so the title is actually a bit of a misnomer. It's not so simple as to just walk and get a 6 pack. It takes discipline and dedication - more in the kitchen than in your workouts, though it takes discipline in the latter too, obviously.
Another thing to be reminded of before I go into the body of this is that I had an advantage here, because I have naturally low body fat. As anyone besides those lame infomercials will tell you, abs are not a result of crunches or core exercises for the most part, they come as a consequence of having a low enough body fat percentage - typically around 13% or under.
I've been sitting at about 11% body fat for the year, good enough to get and maintain a 4 pack. I am also blessed with good genetics meaning that I can be quite liberal in my diet and still maintain that 4 pack. Yes, I know the world isn't fair. It's just very hard for me to put on fat.
Recently I've begun walking several miles a day around my fair city instead of commuting by other means. Reasons being that it doesn't cost me anything (who wants to give the MTA any of their money?) and it gives ample opportunities to day game.
Unfortunately I have been slacking in my workouts this month. Yet paradoxically I've also begun to see the contours of 6 pack abs forming. I have not changed my diet in any significant way these past couple of months, nor have I done any serious new core/ab routines.
The conclusion I've reached is that my walking is what caused the change, and walking is a known good and simple way to burn calories and ultimately lose body fat. I suspect I might have dropped down to 10% or so now - not a huge difference, but enough for the contours of a six pack to display themselves. Whether this will remain as the winter comes closer (and hence, the outdoor walks pause until spring) remains to be seen, but knowing my body, I suspect that it will.
So what does this mean for you, the reader, if you want visible abs but don't yet have them?
If you're heavier fella you're just going to need to change your diet. My cousin, Jason Manenkoff, a competitive powerlifter who owns Iron Arena Power & Performance in Hoboken, New Jersey, has recommended the Renaissance Diet by Mike Israetel to his clients, which I am consequently planning to try over the coming months, as my diet has not been very coordinated.
If you're like me and already lean, you should be able to see your abs. If you can't see your abs, you are too skinny, like I was before. Do some ab and core exercises because you need to pack on a bit of muscle mass. I would then recommend you incorporate some long walks of a couple of miles a day into your routine to shave off that last bit of body fat needed to get that six pack. Tell me if it works for you. I'd be very interested to know.
Also, Mike at Danger & Play has great information on the secrets to abs.