Add 5 mile to your week (so go from 55 to 60), stay there for a month or two or three to fully adapt, then add 5 more miles to your week, stay there for a month or two or three …. Repeat
Once at 60+ miles a week, the best way to do that is adding in 1-2 short second runs per week, until eventually running 13 times a week.
Once at 13 runs a week mileage increases will be to lengthen your runs a little.
Key will be to do this while preserving (not impacting) your two quality workouts and long run each week. That is why increases are kept small and plenty of time to fully adapt is given.
This is essentially what Frank Shorter advised and did.
You can keep this up until no longer seeing improvements or hitting the max amount of time/energy you are willing to put into your running.