Good advice above, here's something to try to get started: if you have a ride you do often that you average 11mph on, let's say it is an 11 mile ride you do in 1 hour. For the first 15 minutes, ride your normal pace to warm up. For the next 15 minutes, ride harder - same gear and pedal faster or higher gear and same pedaling rate. This is often called the "comfortably uncomfortable" pace.
Then 15 minutes back at your normal pace, then the last 15 minutes (or less, since you should finish in shorter time) at a harder pace.
A variant if your riding is on routes where you have to keep stopping, do the "ride harder" on the longer segments between stops.
Keep doing that, soon you will be stuck at a new average speed, but it will be higher than what you started at!