I too have walked the street's as a time served postie, but never ever had to at a certain destination at a stipulated time .
The 'book' did set out how long each part of the 'walk', or duty for non postal workers, should take, but was far as I knew ,that was an example of how long it was to take , for example ,the 'book' would state leave office o715 by OMV, arrive to start of delivery XX MINS, Delivery span 2:30 , RTN to Delivery office o945, as an example of how long the walk should take.
In my experience, some days were shorter than others due to work load , weather or f you were on a 'float' duty or not, if you were on a 'float' you were moved around the area so that you learnt different routes ,and which streets went with which 'walks', I hated that, but you learnt the town likee back of your hand.
I miss the camaraderie of the GPO, I retired early , coz i earned good money, especially when I moved to night work .
But now I cringe at what my fellow workers are expected to do , in my day, we started early , sorted by hand ,with far less mechanical help that they have now, and shifted far more mail in less time,
Very much a case of less is more!