Hard disagree with this.

The number one reason I see people fail their diet/exercise goals is because they go to big to fast.

Training to do an ultra marathon if youve never run a 5k, swam a mile in open water, or even mountain biked for 5 miles is a sure fire way to get injured. You need to build a base of knowledge, muscle, and nutritional habits or you set yourself up for failure, imo. Not a hard rule, but close.

Crossfit is the last thing I would recommend to anyone, especially a beginner.