Witzend
Went swimming today, and during my usual 20 lengths I saw a young woman swimming on her back - while looking at her phone!! She must have done a couple of lengths like that!
As 20 is your "usual" (well done btw) your body will be accustomed to it now and the benefits could diminish. I know, I sound like a bossy boots but I mean well - just a suggestion: try to switch things up a bit by either speeding up alternate lengths or add a length once a month. It keeps your body surprised (so to speak). I find lengths a bit mind-numbing so I vary the strokes, speed and direction occasionally.
I bet if we had smart phones in "our day" we'd be just as in love with them as some young people seem to be now. Personally I'm glad we didn't but you can't turn the clock back to find out.