> I have paddled with several friends at little more than 4 knots
> for the Vogalonga in Venice, three times this year, and it is a 32
> miles course. Newcomers were able to do the same.

Is there any current on this course? A one knot current is not too noticable while paddling yet it can make it can make a 3 knot speed look like 4 knots and the difference between those two can be huge. The 25% increase in speed would take a 50% increase in strength on stationary water.

This could explain differences between what is "possible" and what actually happened.