Mile Time Getting Slower Instead of Faster

A mile time that's getting slower instead of faster usually traces to one of four causes: accumulated fatigue or overtraining, an unresolved injury or growth-related change (common in teenagers), a genuine fitness plateau after months of the same training stimulus, or normal week-to-week variation being over-read from too few data points. Below: a real case of the growth-spurt/injury scenario, plus how to tell which of the four applies to you.

I am a female high school runner. As a high school freshman I ran a 5:20 mile. As a sophomore I ran a 5:11 mile. My junior year I was sidelined due to a growth spurt.

Now as a senior the best I can do is a 5:28 mile. Is it over for me? Did I peak too soon?

I am about 5 inches taller than when I was a freshman and have had 2 stress fractures along the way. I am fully healed and have trained hard for the last three months, but my times are bad. Any ideas? – Janice, Huntington Beach, CA, USA

Answer by Dom: Janice, it is absolutely not over for you—three months of training after twelve months away from racing is nowhere near enough time to draw conclusions. You’re 17 seconds off your best, which is frustrating, but it’s a gap that a well-structured next six months can close.

Here’s what I’ll cover:

1. Which cause fits your situation
2. Understanding the impact of growth and injuries
3. The road to recovery and improvement
4. Looking ahead: where you could realistically be in 3, 6, and 9 months
5. When a slowing time is a genuine red flag

1. Which Cause Fits Your Situation?

Before we get into Janice's specific case, it's worth running through the four most common reasons a mile time slows down instead of improving. Find the one that matches your situation:

1. Recent growth spurt or return from injury. Time off explains the regression. Expect a multi-month rebuild—this is exactly Janice's situation below.
2. Training volume or intensity increased recently with no recovery weeks. This points to overtraining. Back off for 7–10 days and reassess.
3. Same training block repeated for 8+ weeks with flat results. This is a genuine plateau. It needs a change in stimulus, not more of the same.
4. You're comparing only 2–3 data points. This is likely normal variation. Track a rolling 4-week trend instead of reacting to a single bad race.

Janice falls squarely into the first category—growth and injury—so let's look at what that means for her.

2. Understanding the Impact of Growth and Injuries

Five inches of growth is a huge physical change. Your legs are longer, your stride mechanics have shifted, your tendons and ligaments are working around a different skeleton. It is quite normal for a teenage runner’s performance to move up and down during this period—even without the injuries on top of it.

The two stress fractures are the bigger issue. Stress fractures require complete rest to heal properly, and even once they’ve healed, the surrounding muscles and tendons lose conditioning during that time off. Your aerobic base effectively disappeared. You’re not the same runner you were at 5:11—not yet—but you have the underlying physiology to get there again. You just need to rebuild it methodically.

One thing worth knowing: the body changes that happen during a growth spurt can actually work in your favour once you’ve adapted to them. Longer legs mean a longer natural stride. Runners who come through a difficult growth period and return to consistent training often find they can run faster than they ever did before the spurt. That is a real possibility for you, Janice.

3. The Road to Recovery and Improvement

You have been going strongly for three months. That’s great—and you are only 17 seconds away from your best. I know that’s a big gap for the mile. But you have been sitting still for what I assume is about 12 months. You will find you improve much more over the next 3–6 months.

Given your history of injuries and time off, it’s crucial to build back gradually. Don’t push yourself too hard too soon—the stress fractures suggest your body has already sent that warning once. Focus on building a strong aerobic foundation first. Easy running—genuinely easy, conversational pace—should make up the bulk of your weekly mileage right now. Check my page on Increasing Mileage Safely for a sensible week-by-week approach.

Alongside the running, incorporate strength work. Core strength, leg muscles, and hip stability all support your running mechanics and help prevent re-injury. The key with strength training is the same as with mileage: build up slowly. Start with two short sessions per week and go from there. The Strength Training for Runners page has a good starting framework for you.

4. Looking Ahead: Where You Could Be in 3, 6, and 9 Months

Janice, let me give you something concrete to hold on to. Here’s a realistic progression assuming consistent, sensible training from here:

After 3 more months (6 months total from restart): You should expect to be running close to or at your 5:28 current best in training, with race performance pushing towards 5:20–5:22. Your aerobic base will be genuinely solid by this point—you’ll feel the difference in how quickly you recover between sessions.

After 6 more months (9 months total from restart): If you’ve added steady mileage and introduced some threshold running, a return to sub-5:20 is realistic. That would match your freshman year best. Some runners in your situation would be looking at 5:10–5:15 at this stage—a personal best.

After 9 more months (one full year from restart): With a full year of consistent training—base building, tempo runs, and race-specific speed sessions—the 5:00–5:10 range is a genuine target. The longer legs from your growth spurt could be an asset by then, not a liability.

The key word through all of this is consistent. Breakthroughs happen when you least expect them—usually after a period of base building where nothing seems to be changing. Keep your chin up, stay sensible with the training load, and listen to your body if any niggle appears. You’ve got this :)

5. When a Slowing Time Is a Genuine Red Flag

Everything above describes normal, training-related regression—the kind that resolves with time and sensible build-up. But a few signs mean it's time to check in with a physio rather than just training through it. I use a lower bar than the injury-research threshold here because I'd rather you check in a little early than a lot late.

Watch for: a time drop of more than 5% sustained over two or more consecutive time trials—that's a genuine trend, not a bad day. Sharp pain during running, as opposed to general muscle soreness, is never something to run through. And a slowing trend alongside unexplained fatigue or illness symptoms—feeling wiped out well beyond what the training justifies—deserves a proper check-up.

Janice, none of this sounds like your situation—your regression has an obvious, explainable cause. But it's worth knowing where the line sits for the future.
Best of luck with your training,
Dom

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