You probably won’t get much faster than 2:59 if you are stuck around 18:20, as others have recommended, if you can only manage 40 mpw rn do some 800/mile type speed sessions and try to get yourself into sub 5:00 shape or 2:15 shape, as someone that does middle distance mostly I really like transitioning to a marathon block when I’m used to running fast 200s and 400s, and it makes the threshold/MP work feel a lot smoother. I think a lot of the reason why periodization works so well for most people is the mental side of things.