Is the fence in a fixed position? There is a fairly simple OOTB solution.
If it is, draw a polyline starting at your original point (as in your diagram 01), up to the change in angle, so that it goes a short way up the portion along the fence. Ensure that this is all a single polyline, starting at the point from which the lengthen would start.
Once this is set up, LENGTHEN, TOTAL , 1000, will give you what you want.
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