Possible tranmission issue?

I have a 04 trailblazer and this morning I pulled it out of the drive and started down the street. The weird thing about it was that it was going over 20 MPH without me pushing the gas. Any ideas on this? Should I be worried that the transmission is messed up?

How many miles are on this truck?
Did you noticed the RPM's?

does it take off sluggish?