Running Explained: Tempo, Threshold, Fartlek, Hills, Recovery Runs, Trail Running & Why Surface Matters
Running seems simple.
Put one foot in front of the other and go.
But once you start training with a little more intention, it quickly becomes more layered. Suddenly people are talking about tempo runs, threshold work, fartlek sessions, hill reps, easy pace, Zone 2… and it can feel like everyone knows something you don’t.
The reality?
Every type of run has a purpose.
And understanding why you’re doing each one can make running feel easier, more enjoyable, and often improve performance faster.
Easy Runs / Recovery Runs
These are the foundation of most training plans.
Easy runs build your aerobic base, your body’s ability to use oxygen efficiently and keep moving for longer with less effort.
They’re not meant to feel hard.
In fact, many runners accidentally run their easy sessions too fast.
If your easy days become moderate days, recovery becomes harder and your harder sessions often suffer too.
How hard should it feel?
A simple gauge:
✅ You can hold a full conversation
✅ You can breathe comfortably
✅ You could chat the whole way round
Think:
“I could comfortably run with someone and hold a conversation.”
Benefits
Builds aerobic fitness
Supports recovery between harder sessions
Improves endurance
Increases mitochondrial efficiency
Strengthens muscles, tendons and connective tissue with lower fatigue cost
These runs may not feel impressive in the moment, but they’re often where the biggest long-term gains happen.
Tempo Runs
Tempo runs sit in that middle ground:
not easy… not flat-out… but definitely working.
Often described as comfortably hard.
You’re pushing, but controlled.
How it should feel
✅ Controlled discomfort
✅ Breathing noticeably harder
✅ Sustainable for a period of time
❌ Not conversational
A useful guide:
You can usually say 4–6 words at a time, but not comfortably hold a conversation.
Examples:
20-minute tempo effort
3 x 8 minutes with easy jog recovery
Progression run finishing at tempo pace
Benefits
Improves speed endurance
Improves running economy
Helps maintain quicker paces with less fatigue
Builds confidence at race pace
Threshold Runs
Threshold work sits close to your lactate threshold.
This is the point where effort rises enough that lactate begins accumulating faster than your body can clear it.
Training near this intensity teaches your body to tolerate and process that workload more efficiently.
Put simply:
you become better at holding harder paces before fatigue forces you to slow down.
Benefits
Improves sustainable pace
Delays fatigue
Raises your lactate threshold
Helps performance across 5K, 10K, HYROX, half marathon and longer events
Threshold work is challenging, but controlled.
You should finish feeling like you worked hard, but not like you completely emptied the tank.
Fartlek Running
Fartlek means “speed play.”
It’s one of the most flexible ways to train.
Rather than fixed intervals, you vary pace based on effort, landmarks or feel.
Examples:
Run hard to the next lamp post, easy to the next two
1 minute hard / 2 minutes easy
Push uphill, recover on the downhill
Fartlek is great because it feels less rigid than structured intervals but still develops speed and endurance.
Benefits
Builds aerobic fitness
Improves speed
Develops pacing awareness
Keeps sessions mentally fresh
Makes running feel less repetitive
Hill Running
Hill running is one of the most effective forms of run training.
Hills naturally increase muscular demand and encourage strong running mechanics.
They help build:
leg strength
glute power
calf and Achilles resilience
running economy
strength endurance
They also tend to improve technique naturally through:
stronger knee drive
better foot strike under the body
shorter, quicker stride turnover
Types of hill sessions
Short hill sprints
8–15 seconds
Power, speed and neuromuscular work
Medium hill reps
30–90 seconds
Strength endurance and running power
Longer hill efforts
2–5+ minutes
Aerobic strength and threshold conditioning
Treadmill, Road & Trail Running: Why Surface Matters
How you run matters.
But where you run matters too.
Different surfaces create different demands on the body.
Treadmill Running
Treadmills are incredibly useful.
And often underrated.
Benefits of treadmill running
Controlled pace
Easier interval management
Useful in bad weather
Lower external stress
Softer underfoot for some runners
Great for building confidence with pacing
Because the pace is fixed, treadmill running can also teach rhythm really well.
What treadmills don’t replicate as well
Outdoor running includes variables like:
wind resistance
corners
cambers
uneven terrain
small pace fluctuations
changing gradients
Your body is constantly adapting.
On a treadmill, many of those variables are reduced.
That can mean:
less stabilisation demand
less foot and ankle reactivity
lower proprioceptive demand
So treadmills are useful, but they don’t completely replace outdoor running.
Road Running
Road running tends to feel slightly tougher mechanically.
You’re fully responsible for propulsion, pace control, balance and absorbing impact every step.
Road running develops:
durability
running-specific conditioning
race-specific pacing
impact tolerance
If you race outdoors, road running often becomes important for specificity.
Trail Running & Proprioception
Trail running adds another layer entirely.
Roots, mud, rocks, cambers, uneven paths and constant terrain changes force your body to adapt every step.
This challenges proprioception.
What is proprioception?
Proprioception is your body’s awareness of where it is in space.
It’s how your feet, ankles, knees and hips make small adjustments automatically without conscious thought.
Every uneven landing requires your body to react instantly.
Over time this can improve:
balance
coordination
ankle stability
foot strength
stabilisation
body awareness
reaction time
This is one reason trail running can be incredibly valuable, even if you mainly race on the road.
Why mixing surfaces can help
You don’t need to pick one forever.
Using different surfaces often creates a more rounded runner.
Treadmill
Best for:
controlled pace
intervals
winter training
easy aerobic sessions
Road
Best for:
race specificity
pacing practice
building durability
Trail
Best for:
proprioception
stability
foot strength
variety
reducing mental fatigue
Each gives something slightly different.
How Hard Should Each Run Feel?
A simple guide:
Recovery Run
Effort: 2–4/10
Talking:
Full sentences easily
Easy Run
Effort: 4–5/10
Talking:
Comfortable conversation throughout
Tempo Run
Effort: 6–7/10
Talking:
4–6 words at a time
Threshold Run
Effort: 7–8/10
Talking:
Short phrases only
Intervals / Hard Hill Reps
Effort: 8–10/10
Talking:
Very difficult or not possible during efforts
🔬 The Science Behind It
Different types of running challenge different systems in the body.
That’s exactly why variety matters.
Easy running → Aerobic development
Easy and recovery runs primarily develop your aerobic system.
This improves your body’s ability to:
use oxygen efficiently
produce energy for longer durations
recover between harder efforts
Over time, this builds endurance, improves efficiency, and creates the foundation for everything else.
Think:
👉 better engine, better recovery, more capacity
Tempo & threshold running → Lactate clearance and pace tolerance
Tempo and threshold efforts push you closer to sustained harder effort.
At these intensities your body produces more lactate and metabolic by-products. Training here helps improve your ability to clear and reuse them efficiently rather than allowing them to build up and slow you down.
This improves:
sustainable speed
fatigue resistance
running economy at faster paces
confidence at race effort
Think:
👉 holding faster paces for longer before fatigue takes over
Hill work → Strength and power output
Hill running adds muscular demand beyond flat running.
You’re producing more force into the ground with every step.
This helps improve:
posterior chain strength
calf resilience
power output
stride efficiency
running-specific strength endurance
Think:
👉 strength training through running movement
Trail running → Neuromuscular control and proprioception
Trail running challenges more than fitness.
Because terrain changes constantly, your body is forced to react step by step.
Your nervous system, feet, ankles and hips work together to stabilise and adapt continuously.
This improves:
proprioception
balance
coordination
foot and ankle strength
stabilisation
reaction time
Think:
👉 better movement awareness and stronger stabilisers
Why it all matters
No single run gives you everything.
Easy running builds the engine.
Tempo and threshold improve sustainable speed.
Hills develop strength and power.
Trail running challenges balance, coordination and control.
The combination of these creates a more complete runner.
More resilient.
More efficient.
And often, more enjoyable to train.
Final Thoughts
Not every run should feel hard.
And not every run should look the same.
Some days running should feel relaxed and conversational.
Some days controlled and uncomfortable.
Some days playful.
Some days powerful.
The best training plans use variety, with purpose.
Understanding what each type of run does helps you train smarter, recover better, and enjoy running more.
Because “going for a run” can mean a lot of different things…
and each one has its place.