Endurance vs Speed: What's Better for Your Running?

I am doing a couch-to-5k program. I'm a 46-year-old woman, non-runner. I'm now up to 25 minutes of running with no rest, but I go really slowly — maybe 2 miles in that time. Is it better to maintain endurance and not have to rest, or to run faster but have to take rest breaks? I'm training for a 5k race.

– Cathy, Albany NY

Answer by Dom: Cathy, you're doing this exactly right. Stay slow and keep going non-stop: that non-stop running is doing far more for you right now than any speed work would.

This comes up constantly, so I want to give you a proper answer. Here's what I'd cover:

1. Why the 5k is an aerobic event: not a sprinting one
2. What each running speed actually trains
3. Why speedwork too soon is a shortcut to injury
4. What to do after your first 5k

The 5K Is Almost Entirely Aerobic

Here's something that surprises a lot of us who are new to running: most estimates say the 5k relies on your aerobic system for roughly 85–90% of its energy demand. Even elite athletes (people finishing in under 14 minutes) are working aerobically for the vast majority of the race. So for you, covering 5k at a comfortable pace, the aerobic contribution is even higher.

That matters because it tells you exactly what to train. Your aerobic system is built by easy, steady running: the kind you're already doing. Running slowly for 25 minutes non-stop is building your cardiovascular capacity, strengthening your connective tissues, and teaching your body to use fat as fuel. That's not "just jogging." That's the foundation of everything.

Speed work (intervals, tempo running) trains a different system. It's useful and important later. But right now you don't yet have the base to make it effective, and that's perfectly fine. You've been running for a short while. Aerobic base takes months to build properly.

What Each Running Speed Actually Trains

It helps to understand what's happening in your body at different paces:

Easy running: conversational pace, where you can speak full sentences without gasping. This builds your aerobic base: more of the oxygen-burning machinery in your muscle cells, better blood delivery, stronger heart. The foundation of all running fitness. See the base running drills page for the detail on why this matters so much.

Tempo pace: a "comfortably hard" effort, roughly 30–60 seconds per mile faster than your easy pace. This pushes your lactate threshold (the pace where your legs start to burn and slowing down gets hard to avoid): the point at which your legs start to feel heavy and your breathing becomes labored. You can sustain it for 20–40 minutes but you wouldn't want to chat.

Interval pace: faster still, done in short bursts with recovery between. This trains your VO2 max (the most oxygen your body can use per minute, a measure of aerobic fitness): how efficiently your body delivers oxygen to your muscles. Very effective. Also the most demanding on your body, and the most likely to cause minor aches if your base isn't solid first.

Each type builds on the one before it. You can't build a sturdy roof without first laying the foundations. I'd rather you arrive at your 5k healthy and well-prepared than get injured chasing a faster training pace you don't need yet.

Why Speedwork Too Soon Causes Minor Aches

There's a sensible reason beginners are advised to avoid speed work early on, and fitness is only part of it. Your tendons, ligaments, and bones adapt to running stress more slowly than your cardiovascular system does. Your heart might feel ready for faster running before your joints are.

Faster running dramatically increases the forces on your knees, ankles, and hips. Without adequate base mileage to condition those tissues, interval sessions are a reliable way to pick up an injury that sidelines you for weeks. Shin splints, IT band issues, plantar fasciitis: these are often linked to doing too much too soon.

Staying at easy pace keeps injury risk low and lets you build consistent mileage. Consistent mileage (week after week, month after month) is what actually makes you fitter. A sensible training plan with no injuries beats an ambitious one cut short every time.

You're currently covering about 2 miles in 25 minutes, which puts your pace around 12:30 per mile. That's a perfectly healthy easy pace for someone building from scratch. Your target for the 5k is simply to run the whole thing, and you're already very close to doing exactly that in training. :)

After your race, keep building your easy running. I'd suggest increasing your long run gradually, adding roughly half a mile every couple of weeks, until 5k feels genuinely comfortable, then push on to 4 or 5 miles. At that point, if you want to get faster, then we start talking about tempo runs. But that's a later conversation.

Keep going. You're doing brilliantly.

Kind regards,
Dom

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