Stick with the CrossFit work out regimen and build up your riding endurance. Your overall workout should be geared more toward endurance than strength. Your riding should concentrate on endurance also. Try to ride 3-4 hours at a fast pace you can maintain. Work on increasing the pace while maintaining it for the entire ride. Your stamina and riding skill will increase.
I'd look at running BITD Vegas to Reno for good desert practice. It will will also give you a gauge of what you need to work on as a rider.
As far as mechanical skill, if you real have no ability and don't have a natural sense on how things work find someone that can help you. Start with basic things like changing the oil and tires, then start disassembling more parts of the quad just to get familiar with how the pieces come apart and go back together.