This has been my norm for most of adult life, and I recently discovered Chaclas which kinda feel like a cross between Birkenstocks and Teva and are a breeze to walk 15 to 20 miles in. Highly recommend checking out their sandals
I have a pair and as an avid hiker and trail runner... I'm not sure I'd walk 15-20 miles in them. I have other better-suited footwear for that!
I personally have walked 10 miles in a pair, but it was pretty casual walking.
Get them on sale, and send them in to get refurbished from time-to-time. They are fantastic.