I finally discovered last weekend how to show the distance to a next waypoint. Today I did a test. My first waypoint should have been at a bit less then 3km, but I had to run 400m further before arriving at the first waypoint. I also noticed on the website that the first km is only 450m from the start. Taking into account that fact, that might explain the difference but on the other hand I would think that a waypoint has specific coordinates and not a specific distance on the route. Meanwhile I made the route once more directly on GarminConnect. The first one was made on another site and then imported on GarminConnect. Now the first km is at the right place and the distance to the first waypoint is around 400m less then with the first route. So it seems to be in order.
But in this case I could make the route on the site. But what if I can't (a GPX from a raceorganiser with offroad parts)?
PS I have a Fenix 5X