raising average speed

bluebarnsta Posts: 7
edited September 2011 in Road beginners
I'm planning to do a 100mile sportive at the end of the month and have done a couple of long rides as training. My question is how to raise my average speed from 15mph to maybe 17/18mph? I can ride my local route of 18.2miles in just under an hour, but over the last two 100mile+ rides i just average 15mph. I have recently started to spin more which i believe is more efficient but im not sure if it is a lack of stamina, muscle or what.
any help or suggestions as to how i can increase my speed id be grateful.