I'm Fit, So Why Is Running So Hard?!?!

During my wife's first half-marathon she wound up running alongside a woman who was obviously fit. It turns out she was in fact a fitness trainer at one of the local gyms. She was also obviously struggling with this race. She looked at my wife and wondered out loud how this could be so hard when she was already fit. She reasoned that as good shape as she was in she should have been able to complete this race without this much suffering.

I've heard the question. I've even watched the question formulating in a runner's mind. I knew what they were going to ask before they knew what they were going to ask. "If I'm this fit, why is running so hard?"

The trainer's regimen in preparing for the half marathon included precious little running. She mainly participated in and led fitness classes. She thought that this would prepare her for the half marathon. There were two problems with her approach.

1. Specificity. If you want to train for a long run, you should do some long runs. It is obvious that weight lifting alone will not prepare one for a race, but it is less obvious with other exercises. Similar exercises are better, but exact exercises are the best.

2. Energy systems. Other than training your legs to run, you also must train the correct energy systems.

The first energy system is the phosphagen system. This system is lousy for distance running. This is the energy system used for short intense bursts. It lasts for about 10 seconds. This is the system used when lifting a heavy weight or a short sprint.

Second is glycolysis. It produces energy less quickly than the phosphagen system, but lasts longer. It is still not the best for distance running as it only lasts from 30 seconds to about 2 minutes. My suspicion is that the aforementioned trainer did a lot of interval workouts with intervals lasting in that range. She probably had this system well developed from her workouts.

The final energy system is the missing piece of the puzzle for someone who does a lot of fitness, but struggles at a longer run like a 5K (or sometimes even a mile). The aerobic system. The power of the aerobic system is that it can provide lots and lots of energy. It just takes does it more slowly, and unlike the other two systems, requires oxygen. It actually provides 18 times the energy per molecule of glucose as glycolysis does. Not to mention the main fuel source of the aerobic system is fat, rather than carbohydrates as in glycolysis. There is a rather limited supply of blood sugar and stored sugar in the body as compared to fat stored in the body.

Of course, the body is complicated, and it uses energy from multiple systems at once so the faster one is going, the more glycolysis starts to kick in. Even given the fuel necessary to keep the engine running, the chassis has to hold together through the duration, but the point of it all is this.

I'm still learning to swim in the aerobic zone.
This is what the upper limit of glycolysis
 looks like in a triathlon swim.

To run long distance the aerobic system must be developed. Working out with glycolysis as your fuel source is great it burns more calories in the same amount of time, but if that is someone's only way of working out what usually happens in a long run is that they try to run the way they workout.

It happens this way. Fit person is accustomed to a certain level of perceived exertion, runner is used to running long distances. Starting gun goes off. Runner takes off at an aerobic pace, fit person takes off at the RPE (rating of perceived exertion) they usually workout at. Two minutes later, runner is still going at the same pace, but the fit person has reached the limit of glycolysis and starts slowing down. At this point they either gut it out and keep running (albeit more slowly) and never really recover their breath, or they slow down enough to recover and do it again. Meanwhile, runner keeps going at steady pace.

So, it pays to develop all three systems. This means you too all you Long, Slow, Distance runners. Running uses all three systems to different extents so don't forget to train the other two systems as well.