That's a great time, you're off to a cracking start!
Training plan.... map out a few different routes around your area. Start at your current 30 miles, and if you've got energy to spare at the end of the ride, make the next one 5 miles longer. If you can maintain a 15mph pace for 30 miles, you'll probably get up to 50 mile distance (maybe a tad slower) without much problem. Then it's time to go exploring - by trial and error you'll soon discover some favourite routes and maybe some new bits of countryside you've never been to! Don't get too hung up on average speeds and whatnot, just put the miles in and enjoy the riding, from the sounds of it you will soon be irritatingly fast