Certainly sounds as though the clutch is slipping, you should be able to hear the revs increasing with out the speed going up. Check to see if there is any free play on the clutch pedal, can you lift it up or is it against the stop? If there is play and its engaging in the normal position, you need a new clutch I'm afraid. If you drive carefully so that it doesn' t slip it could last a long time but the more it slips the quicker it will wear.

The clutch is more likely to slip at speed than stationary firstly because you have more torque at higher revs and I believe that the centrifugal force on a rapidly rotating clutch encourages it to slip.