Start by defining what you want to extend
“Swim longer” can mean several things: finish 1,500 m without feeling emptied, hold a set of 100s more consistently, or progress from 50 m efforts to 200 m repeats without losing your stroke.
Define the limit you want to move first. That prevents “more total volume” from becoming the automatic answer.
Lengthen part of the workout, not the whole workout
If 10 × 100 m is comfortable, try 5 × 200 m at a similar effort with enough recovery to keep the swimming clean. The set is still 1,000 m, but you spend twice as long swimming between walls.
Another option is to keep the 100s and trim the rest slightly. The work becomes more continuous without adding a metre.
Information
A four-step example
Step 1: 10 × 100 m, 20 s rest. Step 2: 5 × 200 m, 30 s rest. Step 3: 5 × 200 m, 20 s rest. Step 4: 2 × 400 m + 2 × 100 m, all at controlled pace.
The main-set volume stays close to 1,000 m. What changes is continuous swimming time and your ability to manage pace.
An overambitious pace can look like poor endurance
If every repeat starts like a test, the problem may be pacing rather than endurance. To extend the effort, look for a pace you can repeat without a growing time drop-off or major technical breakdown.
You should finish knowing you worked, not feeling as though you survived the final two repeats. Holding a steady pace is itself an endurance skill.
Change one variable at a time
Avoid lengthening repeats, cutting recovery sharply, and increasing total volume in the same week. You will not know which constraint caused the session to fall apart.
A modest change is enough: a few longer repeats, five seconds less rest, or a slightly longer main set. When your swimming remains stable, progress the next step.