The 2026 fall marathon calendar — Berlin (28 September), Chicago (12 October), New York (2 November) — has its registration peak behind us, and the men who hold those bibs are now in the foundation month of training. May determines whether a marathon goes well or badly. And the most common mistake in May is choosing the wrong workout type for the build phase: intervals when tempo runs would do more work.
Why men default to intervals
Intervals (typically 4-6 x 800m or 1000m at 5K race pace with 90-second jog recovery) are popular because they feel like work. You finish, you've sweat, you've struggled. Tempo runs (sustained 20-40 minute efforts at lactate threshold, roughly 80-87% of max heart rate) feel comparatively easy — you're not gasping, you're running fast but controlled. Most men, particularly those who came to running from team sports backgrounds, default to "harder = better" thinking and skip tempo runs.
What the physiology actually says
Three months of foundation training has different goals from the sharpening phase two weeks before race day. In the foundation phase, the dominant adaptations are:
- Mitochondrial density increase in slow-twitch muscle fibres — built primarily by easy aerobic running (60-75% max HR) at sustained durations of 60+ minutes.
- Capillarisation of running muscles — built primarily by tempo work and aerobic running, not by intervals.
- Lactate clearance threshold elevation — built almost exclusively by tempo runs at lactate threshold pace.
- Glycogen storage capacity — built by long runs at slow pace, 18-24 km in length.
Note what's not on the list: VO2 max. VO2 max is the primary adaptation target of intervals, and it is the LAST adaptation you should be targeting in May for a September-November marathon. Intervals in May produce fitness that peaks in 6-8 weeks and is gone by race day. Tempo runs in May produce fitness that compounds for 16-20 weeks.
What a proper May 2026 base-building week looks like
For a male amateur runner targeting a sub-4 hour or sub-3:30 hour marathon, training 4-5 days per week in May:
- Sunday: long run, 18-22 km at conversational pace (75-80% max HR). Should feel slow. If you can't talk through it, slow down.
- Tuesday: tempo run, 25-35 minutes at lactate threshold pace (about marathon pace + 10-15 sec/km, or 5K race pace + 20-25 sec/km). After 20 minutes you should feel "comfortably hard" but in control.
- Thursday: easy run, 8-10 km, conversational. This is the recovery run that most people skip — and skipping it is why their hard days are inconsistent.
- Saturday: medium-long run, 12-16 km, mostly easy with a final 4-6 km at marathon pace. The marathon-pace finish is the only specifically race-pace work in May; everything else is sub-marathon-pace.
What does NOT belong in May
Two workout types should be limited or absent in May:
- VO2 max intervals (3-5 minute reps at 5K pace) — save for August-September.
- Race pace 5-10K sustained efforts — these specific workouts peak benefits 2-3 weeks before the goal race; doing them in May is wasted work.
The Norwegian double threshold approach
If you're a more advanced runner (sub-3:00 marathon, 5+ years of consistent training), the Norwegian double threshold approach — popularised by Ingebrigtsen brothers' training and now widely adopted — substitutes for the single tempo run model. Two threshold sessions per week, each subdivided: e.g., Tuesday morning 5x 6min at lactate threshold, Tuesday evening 5x 6min at slightly faster threshold. The Bakken protocol used by the Norwegian elite athletes works for amateurs with 60-90km/week base. Below that volume, single tempo per week is more sustainable.
Heart rate targets that actually work
If you train by heart rate (most accurate for men over 35), the May 2026 target zones for a runner with 180 max HR:
- Easy/aerobic: 126-144 bpm (70-80% max). 70% of weekly mileage here.
- Steady aerobic: 144-153 bpm (80-85% max). About 15% of weekly mileage.
- Tempo/threshold: 153-162 bpm (85-90% max). 10% of weekly mileage.
- VO2 max+ interval: 162+ bpm (90%+ max). Less than 5% of weekly mileage in May; 8-10% in August.
What to monitor in June to know it's working
Three indicators confirm base building is on track:
- Easy run pace at the same heart rate is gradually getting faster — if your 144 bpm pace was 5:45/km in May and is 5:30/km in late June, your aerobic base is improving exactly as it should.
- Tempo runs feel more sustainable at the same pace — by late June, a 30-minute tempo at the same pace as your May effort should leave more in the tank.
- Resting heart rate trending down 2-4 bpm — a sign of aerobic adaptation, captured by any consumer watch.
The marathon is built in May and June, not September. Most men with autumn marathon bibs are doing intervals when they should be doing tempo. The September race result will reflect that choice.