A sub 2:20 marathon is equivalent to a 30 min 10 k. So imagine you have to run faster than your current 1500 m pace for 10 k. It's a long way to go...
You don't just have to improve your endurance level with a lot of mileage (slowly increase to 100 - 130 miles per week) and a good amount of threshold work (long intervals, tempo runs, fast finish long runs every week). But you also have to work on your basic speed. A sub 2:20 marathon/30 min 10 k definitely needs 4:00 flat and sub 2:00 1500 m/800 m, too.
This post is not good advice. The OP has zero chance of running 2:20 so to run 100-130 mpw would be a huge waste of time. The guys I’ve known that that broke 2:20 were all 9:20,or faster, for 3200/2-miles in HS.