Broken 3800m Ironman Swim: 4x950m at CSS

Session Overview

This advanced pool session covers the full 3800m Ironman swim distance, broken into four 950m blocks held at CSS (Critical Swim Speed) pace. It builds the race-specific endurance and pacing discipline needed to exit the water fresh and ready to race. Designed for athletes preparing for IRONMAN or IRONMAN 70.3 events.

What You’ll Need

• Pool (25m or 50m)
• GPS watch or poolside clock for pace tracking
• Knowledge of your CSS pace (run a 400m and 200m time trial to calculate if unknown)
• Pull buoy and paddles (optional for the first block)

Warm-Up (15 minutes)

Swim 400m easy freestyle, concentrating on long relaxed strokes and bilateral breathing. Follow with 4 x 50m as alternating catch-up drill and normal freestyle, then 2 x 100m at steady aerobic effort (Zone 2) building to CSS in the final 25m of each rep.

Main Set

Swim 4 x 950m at exactly CSS pace with 60 seconds rest between each block. In a 25m pool that’s 38 lengths per rep; in a 50m pool it’s 19 lengths. The goal is consistent splits — not faster on the first rep and slower on the last.

• Rep 1 (950m): Hold CSS pace. Focus on technique and relaxed breathing. Rest 60 seconds.
• Rep 2 (950m): Hold CSS pace. Check split vs Rep 1 — they should match within 5 seconds. Rest 60 seconds.
• Rep 3 (950m): Hold CSS pace. This is where fatigue tests your form — maintain stroke length even if effort rises. Rest 60 seconds.
• Rep 4 (950m): Hold CSS pace. Push to finish strong over the final 100m — simulate race finish sprint.

Cool-Down (10 minutes)

Swim 400m easy backstroke or mixed strokes at a very easy effort. Focus on loosening the shoulders and returning heart rate to resting. Stretch hips and shoulders poolside before leaving.

Coaching Notes

• If you don’t know your CSS pace, perform a 400m and 200m time trial in the same session. CSS = (T400 – T200) / 2, expressed per 100m.
• If holding CSS for 950m is too hard, drop to 3 x 950m first and build to 4 over several weeks.
• Breathing pattern: aim for bilateral breathing throughout — it builds neck resilience for open water turns.
• Target total session time: 90–100 minutes including warm-up and cool-down.
• This session works best once per week during Ironman base and build phases.