I have run a few half marathons on nothing but short metcon training. Its doable, but you arent going to compete with endurance athletes who have trained specifically for the event. You wanna excel at long distance, you gotta train long distance at some point! I was able to "slog" through 21 as mentally, I now KNOW i can run it, and it just doesnt "hurt" as much as some of the workouts we do, so its purely just a matter of grinding it out. But, as always, knowm your goals. If you want to kick serious tail at half to ultra marathon events, dont expect to do it on metcon and sprints alone! Consult anything written by Tim Noakes.