The prevailing training advice to ‘keep your hard days hard and your easy days easy’ is intended to be simple – but in reality, it can really mess with your head. Most runners understand what ‘easy’ feels like (although whether they have the restraint to stick to that pace is another story). But ‘hard’ is entirely different territory. Just how hard should ‘hard’ actually be? How do you measure it? Is it possible to run a hard run too hard?

To get some perspective, we chatted with a handful of experts about the appropriate intensity for hard runs, signs that you might be overdoing it and all the benefits that you get from pushing the needle.

How hard should a hard run really feel?

One of the issues with the concept of a ‘hard run’ is that it’s subjective. A sprinter may find a three-mile jog challenging, while a distance runner would rather run 10 miles at a steady pace than crank out a handful of track intervals. Then, of course, there are all the factors that can make runs harder than initially intended, like weather, terrain, gastrointestinal issues and how well you slept the night before.

Individual runners’ circumstances and preferences aside, when coaches talk about running hard, they’re usually referring to speed and intensity. A hard run is some kind of structured, faster training session, Jason Fitzgerald, USATF-certified coach and the host of the Strength Running Podcast, tells Runner’s World, which means that it’s faster than the foundational miles that you log at a conversational pace.

Just how fast you run on a hard day depends on the intended purpose of that workout, as different levels of intensity elicit different training adaptations. Essentially, it’s about choosing the appropriate tool for the task at hand.

The ‘hard run’ falls on a spectrum. For example, you might run a 5K at a hard, race pace effort of 7:30 min/mile. However, Fitzgerald explains, a lactate threshold training run, while still ‘hard’, is typically 30 to 45 seconds slower per mile than your 5K race pace. So, you would likely run a lactate threshold run at a pace of 8:00 min/mile. And if your workout calls for you to run intervals at your one-mile race pace, you’d want to push yourself to run at a pace of around 7:00 min/mile, or 30 seconds faster than your 5K pace.

All of these runs are ‘hard’, but they’re specific to the distance and the purpose of the workout.

What metrics should you monitor during your hard runs?

Intention is key when it comes to hard runs, says Kaleigh Ray, certified running coach and exercise physiologist. ‘The purpose of the hard run should be to develop something specific and this is something that runners should be able to articulate before they do their hard run,’ she says.

This is where working with an experienced coach or following a thoughtfully designed training plan can be helpful. That way, you’re not just winging it or pushing yourself for the sake of doing a tough workout.

Applying metrics to your run can also help to ensure that you’re staying on track and in line with your original intention. ‘It can keep runners at the right intensity without doing too much and overloading or doing too little and not getting the stimulus that they’re looking for,’ says Ray.

Fitzgerald is a proponent of using race history to set the pace for hard runs and often encourages beginner runners to register for a few short races after a couple of months of training. ‘That provides a baseline for understanding where their fitness is at,’ he says. ‘If you have a substantial race history, you’re going to know your mile race pace, your two-mile race pace, your 5K race pace – and you’re going to be able to execute workouts [that align with those paces] more effectively,’ he says.

If you don’t have recent race data, you can also keep an eye on your heart rate. ‘If you’re looking at a five [heart rate] zone training model, I would say that a hard run is zone 4 or 5, whereas zones 1 and 2 are easy and zone 3 is like a tempo [or moderate intensity],’ says Todd Buckingham, Ph.D. exercise physiologist at PTSportsPRO in Michigan.

To determine your heart rate zones, first estimate your maximum heart rate (MHR). (Subtracting your age from 220 is one easy method, but there are other equations that may be more precise.) Each heart rate zone is a percentage of your estimated maximum heart rate:

Zone 1: less than 57% of MHRZone 2: 57-63% of MHRZone 3: 64-76% of MHRZone 4: 77-95% of MHRZone 5: greater than 95% of MHR

Alternatively, you can use your own rate of perceived exertion (RPE), which you use to subjectively grade your effort on a scale of one to 10, where one is resting and 10 is a maximal, all-out effort.

Will Baldwin, USATF and VDOT-certified running coach, considers any run that’s at an RPE of seven or higher to be hard. He also notes that you don’t need to maintain a hard pace for the duration of a workout for it to be considered hard. ‘We’ll usually have an easy warm-up, some strides and a few different pace ranges,’ he says. ‘It’s usually segmented and broken up into smaller chunks, including rest intervals. We’ll almost never have a run that’s all hard.’

How can you benefit from running hard?

Besides adding variety to your training and boosting your confidence, hard runs offer some unique training benefits. Among the most important physiological adaptations is improved lactate threshold, which refers to the body’s ability to clear lactate – a metabolic byproduct – from the blood and use it for energy.

When you train at your lactate threshold pace or faster, your ability to buffer lactate, particularly in zone 4 (the threshold zone), is going to improve, says Buckingham. This means that you can run harder for longer without fatiguing.

High-intensity exercise, like running hard, also affects mitochondria, which are located in the body’s cells and convert oxygen to adenosine triphosphate (ATP), the body’s energy currency. A recent study compared the effects of moderate-intensity exercise and those of high-intensity exercise on mitochondria (often referred to as the ‘powerhouse’ of the cell) in 20 healthy subjects. After six weeks of training, both groups saw improvements in mitochondrial content and overall fitness. However, high-intensity exercise appeared to be more effective at driving mitochondrial adaptations – and more/bigger mitochondria means more available energy.

Lots of research also shows that high-intensity interval training (definitely considered a hard run), can help to boost your VO2 max, or the measurement of maximum oxygen intake and use during cardio exercise. For example, one meta-analysis found that just four weeks of HIIT workouts, with 30-second intervals and about five minutes of volume, increased VO2 max. And increasing the duration of training, the length of the intervals and the total volume led to even greater effects.

Running fast will also develop muscular power, which you need to charge up a hill or sprint across the finish line. And while it may seem obvious, you have to practice running fast to be able to run fast. ‘The faster you run, the more of an athletic movement it is,’ says Fitzgerald. Hard runs, he explains, will improve your turnover and your coordination.

How do you know if you’re running too hard?

Each type of hard run demands a specific level of speed and intensity, but it is possible to run too hard, no matter what kind of workout you’re doing. Here are three obvious signs that you need to take things down a notch.

1. Your form breaks down

If picking up the pace is wrecking your posture, messing with your gait or creating movement inefficiencies (think: flailing arms), it’s not worth it.

‘I hear a lot of runners articulate this as feeling ‘out of control’ – we want to feel like we’re going fast, but we still have control over our legs and that we’re still running tall and strong,’ says Ray. A breakdown in running form can hasten fatigue and ultimately lead to injury, so only go as fast as your brain-body connection can handle.

2. You can’t finish your workout

If you burn out three-quarters of the way through a workout, this is a sign that you’re pushing yourself too hard. The same goes for wildly inconsistent splits or intervals. ‘If I give you [an interval workout of ] 10 reps of 20 seconds, you should be able to finish the last one at the same pace that you finished the first,’ says Buckingham.

If, for example, you run the first few intervals at a pace of 7:00 min/mile and the last couple at a pace of 9 min/mile, he says that you’re probably going too fast for the initial intervals.

3. You’re not recovering

‘If you do a hard day and then you need to take three days off, that’s a huge red flag – that was way too much and too hard,’ says Baldwin. Also, if you’re constantly sore or dealing with injuries, you may need to lower the intensity of your workouts.