sweet ! thanks Magso.
so, if I got this right.
set my path backward (done)
which returns an array of points.
which i can append a new point to a specific pixel distance.
Im reading the doc that you link but I can't seems to find how to "mesure" the distance in px from on that path.
Adding a point seems fairly easy, i will need to figure out how to get the position.x/.x from it to then "teleport" my unit on it.
EDIT :
here is a quick graph to make sure we're on the same page :D

just realized that the link was for 3d. i read the 2d and I found this :
Vector2 interpolate_baked ( float offset, bool cubic=false ) const
Returns a point within the curve at position offset, where offset is measured as a pixel distance along the curve.
To do that, it finds the two cached points where the offset lies between, then interpolates the values