I go mountain biking at least 4 times a week for 3-4 hours at a time, with some road biking thrown in there as well (gotta get to the good trails somehow).
- Go to a good bike specialty shop, ask them to recommend something for about $500, and buy it. May seem like a lot of cash, but having a good bike makes a huge difference. While you're at it replace the tires with some good ones, because they make a huge, huge difference in handling and power. Grippy, wide tires are kind of noisy on the road, and produce a bit of vibration, but it's not a problem.
- Get a good pair of gloves. They're essential For long rides. They absorb vibration and sweat and prevent your hands from slipping around on the handlebars and getting blisters.
- Ride with a friend. Having someone to talk to and shit relieves boredom and makes for a much more enjoyable experience.
- Find some good trails that take a bit of skill to get through. They're very fun and provide a better workout.
- Get a pair of spandex bike shorts, because that way there's nothing to get stuck on your bike parts or your surroundings. Plus women look fucking sexy in those.
At the very least find some old clothes and use them only for biking or other physical activity.
- Take breaks whenever you get tired. Drink some water and sit down for 10 minutes. Also, try to avoid doing something like riding up a huge, long hill that's going to sap all of your energy and leave you feeling like shit and not wanting to ride anymore.
- Wear a helmet.