Open Water 4km Endurance Swim

Session Overview

A continuous 4km open water swim designed for Ironman and long-course athletes building their swim base. This session develops pacing discipline, breathing control, and the mental resilience needed to sustain effort over a full race-distance swim in open water conditions.

What You’ll Need

• Wetsuit (mandatory in UK open water below 22°C)
• Open water swim goggles — tinted lenses for outdoor visibility
• Tow float / safety buoy — always carry one in open water
• GPS watch with open water swim mode
• Swim buddy or safety boat — never swim alone in open water

Warm-Up (10 minutes)

Enter the water and acclimatise: spend the first few minutes floating and splashing water on your face to adjust to the temperature before starting. Swim 200-300m easy at RPE 3, including a sighting check every 6-8 strokes to establish your navigation pattern and get your bearings on the course.

Main Set

Complete 4km continuously, broken into mental segments. Aim for a consistent stroke rate throughout — avoid going out too hard in the first kilometre. Target a pace 10-15 seconds per 100m slower than your CSS pace to ensure you can sustain the full distance.

• Km 0-1: Settle into your rhythm at RPE 5-6, establish sighting pattern every 6-8 strokes
• Km 1-2: Hold steady pace, check in with your breathing and body position
• Km 2-3: Stay focused — this is where mental fatigue sets in; count strokes to stay present
• Km 3-4: Allow effort to rise to RPE 6-7 as you build toward the finish

Cool-Down (10 minutes)

Float on your back for 2-3 minutes after completing the main swim. Then swim 100-200m easy backstroke or side stroke to flush out the effort before exiting. Change and warm up immediately after — hypothermia risk remains real even in summer UK water temperatures.

Coaching Notes

• Sight every 6-10 strokes to stay on course — excessive sighting lifts the head, sinks the hips, and adds significant fatigue
• If cold water affects your breathing, exhale forcefully underwater to activate the dive reflex and calm your nervous system
• For a stepping stone: complete this as two 2km efforts with a 5-minute rest if the full distance feels too much
• Check water temperature and venue rules before attempting — always swim at a managed open water venue