Set the speed in the route inspector. There are two ways to do it.
A speed you type in is remembered for that activity. The next hiking route then starts from your figure rather than the generic one. To go back to the default, delete the entry.
The estimated duration is not just distance divided by speed. It is the time your speed implies on the flat. Added to that is the extra time the ascent typically costs for that activity. This is why two routes of equal length can show very different durations.
The inspector also shows an arrival time. This is the duration applied to the current time of day. Adjust the speed and the arrival time moves with it.