If you can maintain form and focus, by all means do the occasional longer swim as a benchmark.
When you do a longer swim, you'll gain far more benefit if you do it with a clear goal and purpose of minimizing change in some critical metric.
I.E. Can you do the following
4 x 100
3 x 200
2 x 300
1 x 400
while keeping SPL within a range of 3-4 strokes AND in your Green Zone.
For a more exacting challenge repeat that with a Tempo Trainer.
Perhaps the first time you do the set, slow Tempo by .02 each time you lengthen repeat distance
E.G. 100s at 1.20, 200s at 1.22, 300s at 1.24, 400 at 1.26.
If you succeed at SPL control, a week or two later try it again increasing Tempo by only .01--E.G. from 1.20 to 1.23.
If you're successful again, next time you do the benchmark keep Tempo unchanged.
Or you might stay at each level until you keep SPL within a range of 3 strokes, rather than 4.
Work at incremental and measured improvement in your capability to swim distance efficiently.
Head Coach & Chief Executive Optimist
May your laps be as happy as mine.
My TI Story