Greg is right. Your glider won't accelerate instantaneously. That's physics 101. Can you say "Inertia"? And the increase in speed you need has to come from somewhere. Can you say "Conservation of Energy?" The energy comes from a decrease in altitude. A quick, unpleasant one, if you are near the ground.
The whole point of this is that hang gliders fly at a speed that is in the same ballpark as windspeeds. Slower, yes, but not by all that much. (A windspeed of 8-12 mph is 1/2 the stall speed of your aircraft!) Slower aircraft need to include the windspeed in their calculations. The faster your aircraft, the less it matters. one can argue about whether a Cesna should worry about it. For a 747, in any reasonable wind, they don't really care. For a hang glider, in any reasonable wind, if you are ignoring wind-speed in deciding your approach speed, you are taking a big risk. And if you are ignoring gust-differential, you are taking a bigger one.
By the way, if there is anyone in the club who knows this, it is Greg Kendall. He helped design and fly the slow-flying solar-electric plane that has been in the news lately. Can anyone say "Stratosphere?"