Hi. I just bought a cube aim 29er hardtail and am a complete beginner but I'm finding that I'm only using a couple a the gears on the bike when I'm on the road. I'm more or less stuck in the highest gear, but it still seems too easy to pedal. Is this normal? Im probably only going about 11-13 mph. The store says it is because it is set up for off road and I will need to buy a new cassette if I want to go faster. Does this sound right? Thanks in advance for your help.