Hi,I have a similar issue here in Austria. Each route planing time is much higher predicted than reality. Example. Hiking around 4km (flat area, easy to go) is predicted for around 1 and a half hour. As you know, everyone can go 4km in around one hour.
So, there are many possibilities to calculate this. One idea could be that you could define a personal factor for everyone. With this I could define in my personal settings, that I can hike 4km in (for example) 50 min or go with bicycle in 10 minutes.
However. It will need a better calculation.