You are where I am (I am 57 and started riding a year ago) and I have not increased my speed all winter, despite riding close to 400 miles a month. Too bad you are not closer, we could ride together.
This spring I am going to work on the speed some. My plan is similar to what chasm suggests. Ride some long rides at the usual pace. Ride short rides pushing it at a faster pace. I have mixed feelings on the intervals as I hate them so much. I am now in southern Iowa where it is somewhat hilly. Instead of intervals I am going to try to work the hills.