I studied Tang Soo Do. My master was a sixth-degree black belt. Most non-karate practioners don't know that for each successive black-belt degree you must wait a corresponding number of years (e.g. to test for third-degree, three years after second degree). If you like cross-fit/intensity training/cardio, there is nothing like sparring (with protective pads) for two minute rounds. One of the most intense competitive moments I ever witnessed was a sparring match between a 40-something CEO and a 20-something natural athlete. I give a lot of credit to Zed Shaw for learning other martial arts disciplines, like languages, each are the same at the core but physically different enough that you have to study hard. The green belt is the weed-out period - to get there requires lots of training and lots of classes. If you make it to green belt, you can become a black belt. But, as my Master was fond of saying, the color of the belt does not matter - a belt is just something of external significance. Becoming better at martial arts, like programming, is all about focused practice, increasingly correct repetition and practice (with a big social aspect, a peer group who wants to get to black belt).
I could jump pretty high and had good flying kicks. To the envy of others and pride of myself, I loved flying through the air and executing kicks. At a post-green belt test board break, I was so excited that I launched myself through the air at the two students holding the two boards. I jumped so high that I missed the boards and my height was such that I missed the CEO's face by mere inches. Almost certainly would have broken bones in his face, but didn't. So that's my SOB anecdote about showing off and not knowing how to control yourself properly.
If you ever get a chance, go to a friend's black belt test. Depending on the discipline, it may be as short as a long morning or a whole day. A marathon of traditional forms, weapons work, and sparring and endurance. The best part is watching the higher-level belts test (by seniority, later) and the kids there with their eyes a mixture of respect and awe. Some parents, yes, treat karate classes as day care but no kid leaves karate class without learning more about personal responsibility.
Not just karate, in other physically-demanding areas, you begin to push the mind harder. "A sound mind in a sound body". And pushing the mind harder is what makes us grow. It's a cliche but you can't understand the passion people feel for something until you try it for yourself. As the immortal Ken Kesey said - 'they may know where it is, but not what it is'. I love being part of subcultures (salsa dancing, karate, triathlon) that have only experience and a willingness to start/fail/learn as a barrier to entry. Sort of like what the News.YC community is encouraging me to finally start doing. To stop worrying about how good at iPhone/XHTML/mobile app development I may be and to just start putting it out there.