18km Long Run with Marathon Pace Progression: Advanced IRONMAN Run Session

Session Overview

This advanced 90-minute long run builds from easy aerobic pace to marathon target pace across 18km. The progressive intensity mirrors the race-day challenge of running progressively stronger off a fatigued bike — developing both the physical capacity and mental discipline required for a strong IRONMAN or 70.3 marathon.

What You’ll Need

• GPS watch with pace alerts
• Race day running shoes
• 500ml water or electrolyte — carry a handheld or use a loop route with water access
• 2-3 gels if running for over 75 minutes

Warm-Up (3km)

Run the first 3km at easy conversational pace (RPE 4-5, approximately 90-120 seconds per km slower than marathon pace). This is not wasted time — it is the most important part of the session for keeping legs healthy across a long training block.

Main Set

Progress through four pace bands across 12km:

• Km 3-6 (3km): Easy-moderate — 60-75 seconds per km slower than marathon pace (RPE 5-6)
• Km 6-9 (3km): Moderate — 45-60 seconds per km slower than marathon pace (RPE 6-7)
• Km 9-13 (4km): Marathon pace — target race day km pace. This is the key training block.
• Km 13-15 (2km): Marathon pace or slightly faster — controlled acceleration to simulate a race finish

Cool-Down (3km)

Return to easy pace for the final 3km. Allow heart rate to fall below 130bpm before stopping. Spend 10 minutes on static stretching immediately after: calves, hamstrings, hip flexors and quads are the priorities after any long run.

Coaching Notes

• This session is best run 2-3 days after a hard bike session to simulate the cumulative fatigue of race day
• The marathon pace section (km 9-13) should feel hard by km 12 — that is the point of the session
• Make it easier: reduce to 14km total with only 2km at marathon pace
• Make it harder: extend the marathon pace section to 6km or add a 2km tempo effort at the end
• Heart rate targets: 125-135bpm warm-up, 140-155bpm marathon pace, 130bpm cool-down