Movement memory often returns before fitness
If you used to swim regularly, technical sensations can come back quickly. You recognize the stroke, remember how a turn feels, and find familiar positions. But the ability to hold those movements cleanly through a whole workout may lag behind.
That is where many returns go wrong: the first lengths are swum according to the memory of an old fitness level, then technique falls apart because shoulder endurance, breathing comfort, and tolerance for repeated work have not returned at the same pace.
Start with a session you could probably extend
For the first few swims, it is better to finish feeling that you had a little room left than to empty the tank. Use repeats short enough to keep the stroke organized, take rest when you need it, and keep most work easy to moderate.
If a continuous 100 becomes messy, return to 25s or 50s. That is not going backward. It is choosing a distance that lets you rebuild quality instead of accumulating compensations.
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A useful early benchmark
You should finish most repeats knowing how you want to swim the next one. If every wall becomes a negotiation about whether you can leave again, the session has probably become harder than intended for a return.
Progress one variable at a time
It is tempting to increase total distance, repeat length, and intensity all at once. Then it becomes difficult to understand what is causing the fatigue. For a few sessions, keep two variables fairly stable and change the third.
You might keep the same 50 m structure and add a few repeats. Or keep the same volume and reduce rest slightly. Or hold volume and rest while adding a few controlled accelerations. That makes the progression easier to read.
Do not judge the return only by your old times
Your old 100 pace may come back, but it is not the only useful signal. Notice whether you can hold a similar stroke count, breathe without urgency, repeat a pace from one 50 to the next, and finish the session with technique that still looks and feels organized.
Those signs show that your swimming is becoming stable again. That stability is what will let you add more volume or speed without turning every practice into an exam.
You do not have to make up for the break
There is no mileage debt to repay. A week or several months away from the pool does not require immediate compensation. What matters is rebuilding a routine comfortable enough that you can come back a few days later.
The best first session is often the one that makes you want to do a second.