Technically it isn't "walking," but I've completed eight 26.2-mile marathons. (This is not a large number in serious running circles.) It's a good bet that my total mileage on foot most of those days topped 27 or 28. I'm sure several FTers have completed at least one longer ultramarathon. (The shortest ultramarathons are usually 50km/31.1mi., though even a race of 26.3 miles would probably be considered one.)
Some of my hikes to the more remote parts of New Hampshire's White Mountains, or the easier ones along England's Thames Path, probably exceed this by a bit but took a lot more time to do it.
My recent high was about 15 miles in Torres del Paine, Chile, with a group that the REI outdoor equipment chain put together. I've had days with higher mileage totals, but the terrain is difficult and very little of it is horizontal. No staying in bed the next day, either.