Your overall time is your overall time and can be directly compared to everyone elses times.

If you raced 450B and had 4 laps in at 1:18:06, and someone in the 450A had 1:18:30, you beat him and can brag about it.

What Spermy was saying is that the 70min. race time begins when the first row takes off.
ie.
1nd row 70 min race
2nd row 69 min race
3rd row 68 min race
etc.