Ride more often. 3-4x 10-15 miles. 30 miles will come very quickly after that.
When I first started I rode 5-6 days a week, starting at 10 miles and then getting up to 60-70 mile rides once or twice a week. Then I went and popped off a 100 miler one day without ever doing anything close to that. But because I had ridden so frequently the cumulative fitness carried me through (that and eating a lot).