The glider gains altitude by using a thermal, then sets off in the desired direction. In doing so it will slowly lose altitude. Before too much height is lost, the pilot will seek another thermal, gain height in it, then set off on course yet again. So it is by a combination of thermalling and straight flight that long distances can be covered. Glider construction is to very stringent strength standards and are also maintained to the highest standards specified by the Commonwealth Aviation Safety Authority.
Structural failure is virtually non-existent in a properly maintained glider.
In fact, gliders can perform manoeuvres that are totally forbidden in the average powered plane, for fear that the plane would be over-stressed. Some modern gliders have a top speed of up to about 250 kph. A normal thermalling speed is about 70 - 90 kph. Thermals can go as high as 14,000 ft or more. Above 10,000 ft it is compulsory to use oxygen.