Speed is not a very reliable measure since it is so dependent on the conditions (Wind, road surface etc..) I would suggest using a bike computer with a cadence monitor and work to maintain a certain cadence. Once you are able to maintain that cadence, work to maintain the same cadence while pushing a slightly harder gear. Most people try to maintain a cadence around 70-80rpm, so that might be a good place to start. You want to be working hard enough that it feels hard but you should still be able to talk in short sentences (i.e. not gasping for air)