i averaged 16-18mph on the bike during ragbrai. my philosophy: wake up early, get a good workout while on the bike, take good solid breaks in each town and enjoy the day. so while I rode hard and was passing A LOT of people all day long, I still “enjoyed” RAGBRAI. And … I got to the overnight towns with enough time to pick a primo tent spot.
don’t worry about your average speed. it’s only april. when i first started training last april, I was probably about the same. You’ll be amazed at what some time on the saddle will do for you over the next few months. and as others have said, you might consider investing a small amount in rethinking your gear layout for a setup for your slick tires.