You open Pacewright. It says “Easy Run — 40 minutes, RPE 3-4.” Tomorrow it’ll say something different. Next week the whole pattern shifts.
This isn’t random. It’s a decision tree that runs every day, checking a series of conditions in a specific order. Here’s exactly how it works.
Layer 1: What Week Is It?
The first question is structural: where are you in the training cycle?
Build weeks are when fitness develops. Volume and intensity increase according to your plan’s periodization pattern — beginners follow a 2:1 cycle (2 build weeks, 1 recovery week), intermediate and advanced runners follow 3:1 (3 build, 1 recovery).
Recovery weeks are when adaptation happens. Volume drops to 60-70% of your normal load. Some intensity is maintained — a shorter tempo or a reduced interval session — because complete deload isn’t necessary or optimal. The purpose is fatigue reduction, not fitness reduction.
Taper weeks happen before races. Volume reduces progressively — 80%, then 60%, then 40%, then 20% — while maintaining some intensity to keep you sharp. Your fitness doesn’t disappear in 1-2 weeks, but accumulated fatigue does. That’s the whole point of tapering.
Race week is race week. Everything is oriented around performing on race day.
Layer 2: Are You Recovered?
Before prescribing anything, the algorithm checks whether your body has had enough time since your last hard effort.
| Last Workout Type | Hours Before Next Hard Session |
|---|---|
| Easy run | 0-24 hours |
| Long run | 48-72 hours |
| Tempo / Threshold | 48 hours |
| VO2max intervals | 48-72 hours |
| Fartlek | 24-48 hours |
If you did a VO2max session yesterday, today is not a tempo run — regardless of what the template says. Recovery takes priority. The planned hard session moves or becomes an easy day.
This is one of the most common modifications the algorithm makes, and one of the most important. Back-to-back hard days are a leading cause of overuse injuries in recreational runners, because the musculoskeletal system needs more recovery time than the cardiovascular system does. Your legs might feel ready. Your tendons might not be.
Layer 3: Is It Safe?
Four safety checks run on every proposed workout:
ACWR check. Is your acute-to-chronic workload ratio in the safe zone (0.8-1.3)? If it’s in the caution zone (1.3-1.5), the workout may be modified. If it’s in the red zone (above 1.5 or below 0.7), it will be modified.
Spike guardrail. Would this run exceed 110% of your longest run in the past 30 days? If yes, it gets capped. Even if your weekly volume is fine, a single abnormally long run creates disproportionate injury risk.
Long run limit. Would your long run exceed 30% of your weekly volume? If yes, either the long run gets shorter or other runs in the week need to increase first. An oversize long run relative to weekly volume is a structural risk.
Volume increase check. Would this week’s total volume exceed the safe increase cap for your mileage level? Runners under 15 miles per week can safely increase 20%; runners over 50 miles per week should increase no more than 5%.
If any check fails, the workout gets modified — and you see exactly which check triggered the change and why.
Layer 4: Environmental Conditions
If weather data is available, pace targets are adjusted before the workout is presented:
- Heat: Based on dew point, adjustments range from 0% to 9% slower across six tiers
- Altitude: 1.5% slower per 1,000 feet above 3,000 feet
These adjustments ensure that your effort targets remain accurate regardless of conditions. A “Tempo at RPE 6-7” stays the same effort whether it’s 55°F or 85°F — but the pace that produces that effort changes.
Layer 5: The Workout
If everything passes — you’re in the right phase, you’re recovered, all safety checks are green, and environmental conditions are accounted for — the planned workout from the template is prescribed.
Each workout type exists for a specific physiological reason:
- Easy runs build aerobic base and promote recovery. RPE 3-4. This is where most of your running should live.
- Long runs build endurance and teach your body to burn fat efficiently. RPE 4-5. Longer than other runs but slower than everything except recovery.
- Tempo runs improve lactate threshold — the pace you can sustain for roughly an hour. RPE 6-7. Comfortably hard.
- Intervals improve VO2max and running economy. RPE 8-9. Short bursts of hard effort with recovery between.
- Recovery runs are deliberately easy active recovery. RPE 2-3. Shorter and slower than easy runs.
The mix of these workout types follows the 80/20 principle — roughly 80% of your running at easy effort, 20% at moderate or hard effort.
Why Rules, Not AI
Pacewright uses a rule-based expert system, not machine learning. Every decision follows an explicit, testable rule with a documented source — training science research, established coaching methodologies (Daniels, Pfitzinger), or algorithm design decisions.
This is a deliberate choice. Machine learning models are black boxes — they produce outputs that work but can’t explain why. A rule-based system can tell you exactly which rule triggered, what data it used, and what would need to change for a different outcome. That transparency is the entire point.
When Pacewright says “Easy Run — 40 minutes, RPE 3-4,” it can show you the full chain: you’re in a build week, you did intervals yesterday (so today must be easy), your ACWR is 1.1 (safe zone), and 40 minutes fits within your weekly volume target. Every link is visible. Nothing is hidden.