Why am I so slow at cycling?

Today I did my first 20 mile (33km) ride on my hardtail XC bike. I learned how to ride a bike about 1.5 months ago, but I've been riding pretty consistently since I learned. I ride exclusively in the city, it's a very walkable city, but the paths aren't always the best. I did 33km in 2 hours 53 minutes, not including breaks for water or to eat.

I see people saying that 10MP/H (16KM/H) average is a good average to shoot for, but i can't even get my average above 7.1MPH (11.5KM/H), even on shorter rides. What am I doing wrong here? How are people going so freaking fast on bikes in cities?