I was recently reminded, quite painfully in fact, about why running the 5K is an important distance or event for triathletes to spend time focusing on.
My reminder came in the form of a duathlon, which was a 5K run/30K bike/5K run. The pace from the start was fast. About 2 minutes into the first 5K, I looked down at my Garmin and saw a 5:19 pace. The rest of the first leg is a blur to me. Once I regained conscious thought during the bike leg, I couldn't help but think, "Holy crap, that hurt! I need to focus on running 5K!"
My point is not about how fast or slow I can run a 5K, but to simply express my thoughts about how this middle-distance running event is a key for you, the triathlete, to gain and maintain run speed, regardless of the triathlon distance in which you specialize.
First, let's talk about what the 5K is from a metabolic standpoint, and how that relates to how we train as triathletes. The 5K is about 90 percent or so aerobic. This is good news for most triathletes, as we are an aerobic bunch, meaning our races are at or below our anaerobic threshold (I usually refer to this as "at or below Lactic Threshold Heart Rate"). So if we're going to spend time focusing on the 5K run, we still need to spend time building our aerobic metabolism via short, moderate and long distance aerobic runs.
Additionally, we continue working on the other types of run workouts we do throughout a block of training: threshold runs, strides, and fartlek running, as well as biomechanical drills that help make us more efficient runners. So, by maximizing your aerobic metabolism through the run training that you are likely already doing, you are already 90 percent of the way towards improving your 5K split. It's of that additional 10 percent I am writing about here.
To begin training for the 5K you will need to figure out your current best 5K time. If you've raced a 5K recently (preferably on a flat course), you can use that time as your basis. If not, using a pace calculator can give you at least a starting point. I would not recommend that you go out and run a 5K on your own to discover what your pace is. The reason is that it is very unlikely you will get an accurate time for your effort. Our tendency as competitive athletes is to push ourselves much harder during competition then we do while on our own. That would leave your solo 5K split a bit slower than that of a 5K racing 100-plus competitors. Besides, there are literally 100s of 5K races every weekend throughout the country, so there's a good chance of a 5K near you within the next month. Just get out there and race!