The term "ad hoc" means connecting two wireless computers (or other wireless devices) together directly without using a router. It doesn't have anything to do with your situation.
Your USB wireless adapter is capable of up to 54 Mbps wireless speeds when connected to a "G" wireless router. However, your router is a "B" router and is only capable of a max speed of 11 Mbps. When you temporarily saw a 54 Mbps connection, you were probably receiving the signal from a neighbor's router.