Im not so sure about crossfit. I looked at the typical schedule of work outs here in my town. Over the course of a year they don't spend enough time in any one exercise to actually get good at them. I think for the average couch tater American cross fit will take them leaps and bounds. Athletes need specific training. Cross fit is a good GPP thing.
I keep hearing stories of non crossfitters joining crossfit compititions and winning, of cross fitters doing endurance racing and doing badly ..
Not bashing, I think that it would be a good introduction to fitness for a typical Couch tater. It will point them in the right direction but sooner or later he will need to do specific training.
I recommend a stationary bike. Less impact on the knees then running.
Joshua W. Worman. My motto for losing weight. "Defeat is a lie, there is only adversity, from adversity I gain strength. From strength I gain power. From power I get Victory. Through victory my chains are broken."