Definition of extreme -- existing in a high degree; going to great or exaggerated lengths; radical; exceeding the ordinary, usual, or expected
In a sense, Buddhism is extreme. Realizing the ephemeral nature of the self certainly exceeds the ordinary, usual, or expected for example. But let's talk about going to extremes in practice.
Mumon, Nathan, and Brad Warner have been discussing extremes in practice. The discussion was inspired by the three-year Tibetan retreat tradition, although the degree to which such a retreat is extreme is, I suppose, a subjective judgment. But as many have pointed out, there is something in humans that makes us want to push to extremes -- climb Mount Everest, swim the English Channel, run marathons.
I can remember when American distance runner Frank Shorter won the marathon in the 1972 Olympic games. Before 1972, in the U.S. marathons were something one read about occasionally in the sports pages, and fitness running was for professional athletes only. After the 1972 Olympics, it seemed everyone wanted to run marathons; or, at least, suit up in the latest running shoes and shorts and go plodding around the block now and then.
There are marathons in Buddhism too, you know. The Tendai school in Japan is known for its extreme running. Monks known as Kaihigyo run 40 kilometers a day around Mount Hiei, wearing white robes and straw sandals, for 100 consecutive days. That's the basic training.
Then the next year or so they run 40 kilometers a day for 200 consecutive days. Eventually a few of them work up to 84 kilometers a day for 100 consecutive days, although I understand most keep it to only 40 kilometers a day. The goal is to run some extreme distance for 1,000 days within a seven-year period.
Extreme? It used to be that after a certain point, monks who could go no further were required to commit ritual suicide, and they were buried on the mountain where they died. That practice stopped sometime in the late 19th century, I believe. Here is an old London Observer article that explains the monk marathoning in more detail.
The point of this, I have read, is to bring monks to the point of death. I have to trust that the Tendai tradition has found this useful in practice. I have yet to hear of any Tendai nuns running themselves to near-death, though.
Nathan writes that some extreme practice appears to rise from good ol' machismo. Yes, probably. As he says, some Zen centers in the West do have a Marine boot camp air about them, although I think there's less of that now than was true a couple of decades ago.
Women are not immune to the lure of extreme, though. Some of the great long-distance swimmers are women. The life of a classical ballerina is pretty extreme, too.
Mumon makes the point that practice really does require arduous effort, but there are ways to weave that effort into day to day life. Don't think for a second that "real" practice means going off to some very different place and doing entirely different things. Your attention to practice is what makes it "real."