Sure, running at the faster pace will get you fitter overall, but it's not going to help you improve in the marathon. This is exactly why your fitness keeps improving and you might run PRs in shorter events, but bonk or fall apart during the marathon.
In short, when your goal time is off, all of your paces are going to be off. That means you'll be running all the wrong effort levels and negating the most important benefit of your harder workouts. You'll be wasting your time training.
More: 6 Ways to Run at the Right Pace
2. Your Risk for Injury Increases
The second major flaw in training for the wrong goal time: it dramatically increases your risk of injury.
Typically, runners will choose a goal pace that is too fast. As such, the balance of hard work and recovery is thrown off, which leads to overtraining. An example:
A tempo run is designed to be a moderate or medium-effort workout. Your training plan therefore assumes that you'll be recovered and ready to run hard again or perform a long run just a couple of days later.
More: How to Execute the Perfect Tempo Run
However, if the tempo run was too fast for you, then the effort level was also increased. This means you won't be as recovered for your next training session as planned. This fatigue slowly builds up throughout the weeks of marathon training until you become overtrained or your muscles, tendons, ligaments and bones give in and you get injured.
More: Prevent Running Overuse Injuries in 6 Steps
3. You Ruin Race Day With Bad Pacing
Finally, race-day pacing is one of the most crucial elements to having a successful race. In fact, studies have shown that running the first mile of a 5K more than six percent faster than goal race pace considerably reduces performance—so much so that almost all of the subjects who ran faster than six percent failed to even finish the race.
In the marathon, running too fast for the first few miles will burn through your glycogen stores faster. This will lead to you bonking and having a terrible race.
More: How to Beat Marathon Fatigue
Once again, we'll use the example from the aerobic threshold run to illustrate this concept.
To hit your goal time of 3:45 for the marathon, you start out at 8:35 or 8:45 per-mile pace. However, if your fitness is currently more like a 4:00 marathon (9:09 pace), then you're already running 20 to 25 seconds faster per mile at the start of the race. Your race will be doomed from the start. It won't matter how bad you want it; you're going to bonk.
More: 4 Valuable Marathon Lessons
How to Find Your Goal Pace
Now that you understand the pitfalls of choosing the wrong goal time, how the heck do you decide what pace you should shoot for?
Step 1: Establish a Baseline
The first thing you need to do is determine your current fitness level. If you've run a race recently, you can use this time to extrapolate what you could run for a longer or shorter distance. Greg McMillan has a great calculator on his site.
If you plan to race the same distance again, no calculations are needed. Simply use this time as your data point for step 2.
More: Why Pace Calculators Can't Always Be Trusted
If you haven't run a race recently that you feel reflects your fitness, you have two options:
1. Race a 5K. This is your best choice if your goal race is more than two months away. The race doesn't have to be big or fancy. You just need a race effort.
2. If there are no races available, complete a 1-mile time trial. This option is recommended if you have 1 to 2 months between now and your goal race because it can be incorporated into training quickly and a mile won't leave you too tired to pick up training where you left off.
Whichever method you choose, just enter your time in the calculator mentioned above and you can extrapolate a suggested time goal for any race distance.
More: Pacing Strategies for Popular Race Distances
Step 2: Factor in Your Likely Rate of Improvement
Now that you have your fitness level established, use your training history to help determine your rate of improvement.
If you've been running less than a year and improving with each race, you can expect about a 6 to 8 percent improvement in performance over the course of your training. As an example, if your data point shows you're in 4:30 shape for the marathon, you'd be looking at a goal time of about 4:08 to 4:13.
If you've been running for more than a year but you're still PRing in most races and are increasing your commitment to training, you can expect a 4 to 6 percent improvement in your performance. As an example, if your data point shows you're in 4:00 shape for the marathon, you'd be looking at a goal time of about 3:45 to 3:50.
More: How to Train for a PR
If you're more experienced and have been training for many years, then you should expect a 2 to 4 percent improvement in performance. As an example, if your data point shows you're in 3:20 shape for the marathon, your goal time should range between 3:12 and 3:16.
More: How Do I Get Faster for My Next Race?
Calculating percentages of race pace and goal times can be difficult, so you can download this calculator if you need help.
Step 3: Adjust and Adapt
Finally, your rate of performance isn't something that is standardized. While the above information provides good guidelines to follow, every runner is different.
After 3 to 4 weeks, if you think you've gotten fitter or you want to measure your rate of improvement to determine if you're making progress towards your ultimate goal, run another race. Try to keep the race as integrated with your training as possible (for example, run the race in place of a hard workout) so you don't impact your long-term progress.
Plug your new race data into the performance calculator from step 1 to see how much your goal pace has improved.
More: Improve Your Run Training: How to Peak at the Right Time
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