NOT OVERNIGHT! 15 mph avg, solo, on rolling terrain/moderate hills is not a geriatric pace and nothing to be ashamed of! Just keep riding consistently and challenging yourself and your might see slight improvements year by year. Don't know if you'll be able to achieve an 18/19 mph avg all by yourself on that type of terrain, but you can get the most of your rides by: pushing the climbs, standing out of the saddle more on climbs until your legs and heart are well used to it. When you get fitter, you may be able to mash bigger gears at an optimal cadence but it's a gradual process. Don't forget to rest/recover. Last but not least, if you can handle it, and only after you have established a good base miles fitness level, try some interval training on flats or hills for several weeks. Hard work but very rewarding; should make you faster and stronger. Also, if it's possible, consistently riding w/ a fit group will make you faster/stronger over time. You might get dropped initially, but stay at it, take your licks, and keep riding. Good luck!