Renault megane scenic 2002 1.6 petrol?

Hi i have a Renault megane scenic 2002 iv got similar problem, start it up and the revs are 3to4000 and it takes a long time to come down to normal running speed, id say when car is very warm. I can nearly get to work and its still running fast, about 4 miles. Any ideas please many thanks.
Jayne.