Are there any workouts you've tried and you just couldn't keep up doing?
Take me for instance, yall remember when I tried to go do crossfit right?
Well...I also did INSANITY! 
ONCE.
Never again.
I'm a PUNK when it comes to full on endurance training and am in AWE of people who can and do it REGULARLY! Whoa! I workout to maintain and will probably be eaten by the Zombies if they chase me for more than 5 miles during the Apocalypse. Luckily I have family, framily and friends who will motivate me by yelling while we're breaking for it. Yes they will let me perish if I can't keep up but I'll understand. Seriously. LOL!
Which ones have you tried and decided to not keep up? LOL!
