BUT next time you go, look for the full-lap stagger in lane 6 and estimate the distance from it to the start//finish line. Multiply that by 3 for your answer. For your purposes, that estimate will be close enough.
Go back and measure the width of a lane, in metres.
Your distance per "600" = 600 + 3 X 5 X 2 X pi X (width in metres)
Each lane of the track increase in distance by 2 X pi X (lane width).
You ran 5 lanes out.
Your 600m repeats had 3 laps each.