I've run 4:28 for the 1600 and am hoping to break 4:20 this year (april or may). what workouts would be a good indicator of sub-4:20 fitness level?
i was thinking:
12x400 averaging 64 with 60 seconds rest
8x600 in 1:40 with equal rest
6x800 in 2:20-2:22 with 2 min rest
or some kind of ladder workout involving these... by the way i run about 60 miles a week.