If a straight line is the shortest distance between 2 points in 2 dimensions is a semi circle the longest line where where each  sucessive point is closer to the final destination? If it  is I have a follow up question

A semi circle is not the longest line between two points where each sucessive point is closer to the final destination. As an example make a circle with the radius from point a to point b. then another circle from another position between those to points, and so on. Every point in the each circle is closer than the points on the outer circles, as the distance to point A is simply the radius. This means you could simply "walk" in a spiral from B to A, going around multiple times, and still have each successive point be closer to A, while having a longer "line" than a semi circle. Thus the starting expressions are wrong. And a successive question is incorrectly based. A drawing would be easy, but i dont have the time now to make one, i hope you understand in any case.

no, in two dimensions, the longest "line" where each point approaches its final destination (assuming the plane traverses infinity) would be an hyperbola (ellipse stretched to infinity).  Additional dimensions just extend the hyperbola: hyperboloid, hyper-hyperboloid, nth degree hyperboloid...