|A bunch of CF regulars this past September
If you do not know what CrossFIT entails, then I urge you to research your area and try the entry level course. In short, CF focuses on total fitness. Core, strength, flexibility, stamina, burst power and sustainable energy. Think of the most well rounded athlete you know, and you have the idea.
I'm fairly fit, but frankly mostly in one dimension - I do massive amounts of cardio work. Ultimately, marathon running is intense aerobic activity. You try to find a comfort zone. When I'm comfortable doing a CF workout, something is wrong.
For over two years, I've been endeavoring to attend CF at least once per week. This falls off during marathon races, but I try to get there twice per week during standard training. We do everything from the obvious like pull ups and push ups to lifting, sprints, rowing, rope climbing, burpees, wall balls and more. The workout typically covers about 30 minutes of technique or warm up or setting a PR for some activity. Then a timed workout follows covering 3 to 4 different exercises. The routines change daily.
Mike Anderson, who I have known for 3 years, opened CrossFit Malibu in September 2009. It's a family/friendly affair with the focus on community and mutual support. http://crossfitmalibu.blogspot.com/ for more info. And if you don't live in this area, I am sure you will find a gym near where you reside.