I'm about halfway through walking the Izumo 33 kannon pilgrimage. There are hundreds of 33 Kannon pilgrimages in Japan, though probably the oldest and most well known is the Saigoku Pilgrimage up in Kansai. The Izumo pilgrimage was founded, like many of the others, around the middle of the Edo period, the time pilgrimages really became popular among common folk.
I havent measured it yet but my guess is it is about 300k in length, possibly longer. Many of the temples are uninhabited and in fairly remote locations. Many are mountaintop temples and therefore there is a lot of ascending and descending to do.
There are some stunning views especially across the OkuIzumo region. There is little in the way of pilgrim infrastructure. I have found no signs along the road and there are no rest huts, though I was very pleasantly surprised to discover that temple 9, Mine-Ji, had a tsuyado for pilgrims. Stores are few and far between, though the route does pass through the urban areas of Izumo City and Matsue City. Occasionaly the route follows main roads, but most of the route is along very narrow, mountain roads with very little traffic.
I recently discovered a map of the old Iwami 33 Kannon pilgrimage!!!!! so once I finish the Izumo 33 that is what I will walk next....... and then the Chugoku 33 Kannon.... and then the Shodoshima 88... and then,,,,