Don't feel obligated to increase your mileage by large increments really quickly. You can gradually up your mileage. If you're comfortable with 10, go for 11 the next time, and 12 the time after that. Slowly the way you think about distances begins to change.

I remember when 15 miles sounded long to me. Any you know what? At that time it was. But it's all relative. You slowly build the confidence and the fitness to ride longer distances. It's not a race to get to 20. You get there when you feel ready.

Good luck!