Why Running Really Irritates Me

Today I ran a 5K race, and was totally disappointed in myself. Recently I completed a Tough Mudder, with 20 obstacles and 10 miles hiking through mud and mountains here in Las Vegas.  As I continue to train for next year's event, I am running in 5K and 10K races throughout this upcoming year to stay conditioned.  Without fail, every time I run a 5K, I hit a personal wall where I can break out and finish strong.  I wonder why seemingly out-of-shape fat people and unlikely running pass me by like I'm standing still.  Either I am too doughy to do these races, or I am seriously overlooking something in the mix.  Help me solve my fitness and running woes, and try to get in shape for 2019.