Your comments only reflect the fact that you don't know how to use a Mercator projection. No one imagines the length scale is constant across the map (the horizontal scale varies as the cosine of the latitude). The point is that North is always vertical (on the map) and you can use that to determine your heading to your next objective point. It will not be a Great Circle, but it will get you there.
If you know the coordinates of two points, you can use spherical trigonometry to find the distance.
Your comments only reflect the fact that you don't know how to use a Mercator projection. No one imagines the length scale is constant across the map (the horizontal scale varies as the cosine of the latitude). The point is that North is always vertical (on the map) and you can use that to determine your heading to your next objective point. It will not be a Great Circle, but it will get you there.
If you know the coordinates of two points, you can use spherical trigonometry to find the distance.