Ever wondered why you're gasping for air after your 5k runs? There's a simple explanation for that my friends. You're just plain as simple, out of air. Seriously runners, as we age our body's ability to use oxygen declines. By that I mean you can't take in enough oxygen to run as fast as you once did. There's a scientific terms for this occurance however, I don't get into that with Runningafterfifty. Just keeping it simple here. Having said that, many older runners gravitate to longer runs. Longer distances don't require maximum oxygen consumption the entire race. Your body can adjust to its oxygen usage. Remember running should be fun. Relax, run your 5k. Who says you have to go all out. And if you do, I know you've trained very hard.