Google maps has a lot of data about how far and how fast you move around. When you search for a walking route to your destination is the app calculating the walking time based on your average walking speed?
It does not take into account your walking speed but assumes a normal walking speed. From Google Maps help
What is the Google Map walking speed? Is this a constant value or does it change depending on slope?
Normal walking speed is about 3 miles per hour - It does not adjust for distance travelled, nor for whether you are on a road or footpath. It does adjust slightly for slope - it decreases more for uphill than it increases for downhill.
A user wanted customized speed, like you do and they were suggested to raise a feature request.