Sub-Threshold Run: 3×20-Minute Aerobic Power Blocks

Session Overview

This sub-threshold run uses three 20-minute blocks just below lactate threshold to build your aerobic ceiling without excessive fatigue. Running at 83–88% of maximum heart rate for extended periods raises your threshold pace over weeks of consistent training — one of the most effective run adaptations for triathlon.

What You’ll Need

• Flat to gently rolling road or path
• GPS watch with heart rate monitor (chest strap recommended)
• Tempo trainers or race shoes
• Hydration if weather is warm

Warm-Up (15 minutes)

Jog 10 minutes easy at Zone 1–2 (conversational pace), then run 5 minutes progressively building from 70% to 80% max HR. Finish with 30 seconds of quick leg turnover drills to prime your stride.

Main Set

Run three 20-minute blocks at 83–88% max HR (Zone 3 — comfortably hard). Take 3 minutes of easy jogging between each block. The effort should feel sustainable but purposeful: you can speak short phrases but not hold a conversation.

• 3×20 minutes at 83–88% max HR (Zone 3)
• 3 minutes easy jog recovery between each block
• Target pace: approximately 10–15 seconds per km slower than 10km race pace
• Total working time: 60 minutes

Cool-Down (10 minutes)

Jog 8 minutes easy, then walk 2 minutes. Stretch your hamstrings, calves, and hip flexors. Allow 36–48 hours before your next hard run session.

Coaching Notes

• Heart rate will drift upwards during the 20-minute blocks — this is normal, don’t panic or slow excessively
• Easier: reduce to 3×15 minutes or extend recovery to 5 minutes
• Harder: extend to 4×20 minutes or reduce recovery to 90 seconds
• RPE target: 6–7 out of 10 — if it feels like threshold (8), you’re running too fast
• Build this session weekly over 4–6 weeks, reducing recovery duration as fitness improves