How can you complete up to Exercise 13 in the space of 4.8 hours? Or did you have previous flight experience?
Providing weather has been perfect and you fly often (i.e you don't need a flight to get you back into flying due to a large lay-off due to weather) then the norm of around 15 hours seems to be correct.
I always raise an eyebrow when people say they have been solo circa 10 hours.