On a good day, it's what has been said already.
On a so-so day, it's a long ride possibly with friends to the point where you wonder if you can go further the next time out. You may be sore the 1-2 days but you'll survive and later wonder what the excitement was all about distance/difficulty-wise.
On a bad day, you question your sanity, possibly lose a great deal of money and/or health (due to an accident, equipment failure or failure of your own body to function properly), and find out that you or your friends aren't the superhero athletes you thought each other was. You're merely human.

That's my take on it.

So much for evangelizing.