A professional bike fitting is well worth the money. Ask the bike shop where you purchased the bike if they do a professional bike fit.
Fuel
Drink before you're thirsty and eat before you're hungry. Some believe you should drink every 15 minutes and eat every 15 miles. Hunger and thirst signals mean that your body is already lacking something. You don't want to bonk it can be ugly. Handy fast snacks include; gels, Fig Newton's, food bars, honey, trail mix and PB&J. This is especially important when riding more than 50 miles.
You really need to pay attention to eating nutritious foods and stay well hydrated in the days prior to a long ride. In addition, make sure you bring more water and food than you think you'll need on the ride. On rides longer than an hour, a recovery drink should be consumed within 20 minutes after getting off the bike.
Pedals
Ditch the flat pedals and toe clips, you will become more efficient when pedaling with clipless pedals. With clipless pedals you can learn to pedal more efficiently. Imagine tracing a circle with your heel: concentrate on pulling up, pushing the pedal across the top, and pulling backward at the bottom as if you're trying to scrape mud off the sole of your shoe.
Gears
Shift your gears. When you slowly push hard gears your leg muscles do most of the work. Spinning smaller gears and pedaling faster transfers the workload to your aerobic system. You'll find a sweet spot where no part of your body feels like it's working too hard. So if you're tired in the legs, shift to an easier gear. If you're tired in the lungs, shift to a harder gear.
Also, remember to clean your cassette at least monthly. If shifting is rough, makes a lot of noise or the chain skips, the cables may have stretched and you will need an adjustment. If you are unsure of how to do this, visit a local bike shop and they will do it for you for a small fee or possibly no fee if you purchased the bike at the shop.
Road Rules
Hold your line, don't cross the center line, ride no more than two abreast, don't ride on the shoulder, obey the traffic signs and signals, ride and act like you are driving a car. Check your state's Department of Transportation for more information about cycling safety.
Increase your confidence at a cycling event.