While I think this challenge is a great idea for those who have been in a fairly consistent exercise routine for a while, I would really caution anyone who has not exercised in a strenuous manner for a long period of time and are seriously overweight to get an OK from your doctor first. While I have been exercising consistently for 2 years, when I first started two years ago, I was lucky to get 5 minutes on the elliptical before feeling like I was going to faint. It took me a good 3 or 4 months of going to the gym 4-5 days a week to get up to 30 minutes. And if I got sick or was away from the gym for a while when I was on vacation, I could never start right back up to 30 minutes of cardio.
I am now doing 40 minutes 4-5 times a week consistently. For me, it is a big accomplishment. And being 35 lbs lighter certainly helps. Along with building up muscle through weight training and core strength and flexibility through Pilates.
As far as yoga is concerned, it doesn't really count as cardio because cardio involves elevating your heartrate to a predetermined level based on your age for a set amount of time. But yoga is a wonderful exercise for flexibility and core strengthening. On new year's day, my gym is offering a yogalilates class (a mixture of yoga and Pilates) which I plan on taking.
Seriously, I could partake in this challenge easily. But I am hesistant to do so because I wouldn't want to encourage anyone to attempt this and then have them suffer a heart attack or something because they pushed themselves too far too fast.