Stop chasing prescribed paces in the heat and learn how to use the PFM Traffic Light system to adjust your workout by effort in real-time.
TL;DR: When temperatures spike, your heart rate and perceived exertion (RPE) become more important than the pace on your watch. If you haven't spoken to me before your session, you have full permission to drop the pace and run by feel. Heat is a massive physiological stressor; forcing a cool-weather pace in 25°C+ is a fast track to burnout. Use the PFM Traffic Light system: if your breathing is ragged or your HR is 15-20bpm higher than usual, move to 'Amber' and ease off.
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The sun is beating down, the humidity is thick enough to chew, and your training plan says "Threshold Intervals at 4:15/km." You look at the thermometer, then back at your watch. You haven't had a chance to message me, or maybe I’m out coaching another athlete and haven't replied yet.
The trap is thinking that the numbers on the screen are set in stone. Many runners feel like they are "failing" the workout if they don't hit the exact splits prescribed. But here is the reality: forcing a pace that was calculated for a 10°C morning into a 25°C afternoon is not "grit" , it's bad system management.
In this post, I’m going to explain why heat changes everything and how you can use the PFM Traffic Light system to make smart, real-time decisions that protect your long-term progress.
Why Heat is a Genuine Physiological Load
When you run in the heat, your body faces a dual challenge. It has to send blood to your working muscles to keep you moving, but it also has to divert a significant amount of blood to the skin's surface for cooling (thermoregulation).
This competition for blood flow means less oxygen reaches your muscles, your heart rate climbs higher for the same output, and your "adaptation budget" is spent much faster. If you try to maintain your standard pace, you are essentially asking your body to perform a much harder workout than intended.
A "Steady" run at 5:00/km in the cool might feel like a 4/10 effort. In the heat, that same 5:00/km might feel like an 8/10. The physiological stress is what matters, not the number on the watch.
The PFM Traffic Light System: Real-Time Edition
At PFM Coaching, we use the Daily Runner Readiness Checklist to gauge how you feel before you step out the door. But when conditions change mid-run, you need to apply that same logic to your effort.
Here is how to use the Traffic Light effort system while you are actually out on the road or trail:
🟢 Green: Everything is Groovy
- The Feel: You are sweating, but your breathing is controlled. You can still speak in short sentences.
- The Data: Your heart rate is within 5-10bpm of your usual zones for this effort.
- The Action: Continue as prescribed. You are absorbing the load well.
🟡 Amber: Proceed with Caution
- The Feel: Your breathing is getting ragged before your legs feel tired. You feel "hot under the collar" and the effort feels 2-3 points higher on the RPE (Rate of Perceived Exertion) scale than it should.
- The Data: Your heart rate is 15-20bpm higher than normal for this pace.
- The Action: Adjust immediately. Drop the pace by 20-30 seconds per kilometre. If it's an interval session, extend your recovery periods. The goal is to bring your breathing and HR back under control.
🔴 Red: Stop and Re-evaluate
- The Feel: You feel dizzy, nauseous, or have a pounding headache. You’ve stopped sweating or feel "chilled" despite the heat.
- The Data: Heart rate is spiking into "red zones" during what should be an easy shuffle.
- The Action: Stop running. Walk, find shade, and get fluids in. This is no longer a productive training session; it's a safety issue.
Your Real-Time Decision Tree
If you find yourself in the heat and the workout feels too hard, follow this simple logic:
- Check your breathing: If your lungs are burning before your legs are aching, you are likely overheating. Ease off.
- Check the "Feel vs. Number": If the plan says "RPE 6" but your watch says you are hitting the pace at an "RPE 9", listen to the RPE. Slow down until it feels like a 6 again.
- The 15bpm Rule: If your heart rate is consistently 15-20bpm higher than it usually is for this specific pace, the heat is winning. Back off.
It is perfectly okay to walk. In fact, if you started too fast and blew up, walking to bring your core temperature down is the smartest thing you can do.
Permission to Adjust (Even Without a Call)
As your coach, I would always rather you run by feel and finish the session healthy than chase a specific pace and end up in a hole for three days.
Personalised coaching isn't about following a static PDF; it's about a human relationship where we adapt to the conditions on the ground. I build your bespoke online coaching plans around your life and your environment. If the environment changes (like a 30°C heatwave), the plan must change too.
You have my full permission to make the call. If you adjust a session because of the heat, just leave a note in your training log. Tell me the temperature and how your RPE felt. We can then use that data to prep for future hot races.
Managing the "System"
Training is about system management. You have a finite amount of energy and recovery capacity each week. If you "burn all your matches" trying to hit a pace in the heat, you won't have anything left for the rest of the week's training.
Make sure you are supporting your system with proper hydration. I often use Torq (paid link) #ad for electrolyte replacement during these sessions. And if you are heading off-road to find shade, a reliable GPS like the Garmin eTrex 32x (paid link) #ad can help you navigate without the stress of getting lost in the sun.
Key Takeaways
- Effort > Pace: In the heat, your RPE and heart rate are the only metrics that matter.
- The Traffic Light System: Use it mid-run to decide when to push, when to caution, and when to stop.
- Physiological Stress: Heat adds a massive invisible load to your body. Respect it.
- No-Fluff Grit: Being a "tough" runner means making the smart decision to adjust so you can train again tomorrow.
- Coach's Permission: You don't need to wait for a text back. If it's too hot, slow down.
FAQ
Q: How much slower should I go?
A: Usually, for every 5 degrees above 15°C, you can expect to drop 10-20 seconds per mile. But don't get hung up on the math , go by how hard you are breathing.
Q: Will I lose fitness if I slow down?
A: No. You are still getting the aerobic stimulus. In fact, you'll gain more fitness by completing a controlled, heat-adjusted run than by blowing up and being unable to run for two days.
Q: Should I do my intervals on the treadmill instead?
A: If it's dangerously hot (the "Red" light on our scale), then yes, moving indoors to the AC is a very sensible "system management" choice.
If you’re struggling with the heat or want a plan that actually adapts to your local climate and life schedule, get in touch with me here.
Happy Running.
