A punishing hill climb is worth it for the view from the top.
When you're pushing your muscles to their limit, you're activating physiological changes in the composition of your muscle cells and in the rate of muscle repair, which causes an increase in size and strength over time. Cycling is a great strength-building exercise for the leg muscles, and there's no way to push yourself harder on a bicycle than on some steep hills.
Leg Muscle Lowdown
Your legs are your primary source of power on a bicycle. The quadriceps and gastrocnemius are two very robust muscles that contain a lot of fast-twitch muscle fibers, giving your legs the power to propel you and your bicycle uphill. Since these muscles are some of the largest in the human body, your cardiovascular system has to work hard to provide enough oxygenated blood for your cycling workout, making an uphill ride a whole-body experience.
Build Up Over Time
Building your muscles is something that happens naturally as a response to vigorous exercise. Hypertrophy occurs when you exercise, which is a natural expansion of the individual cells in your muscles. This cellular expansion makes your muscles faster and stronger, and can occur after just a few days of exercise. As you work out a muscle, you'll also cause micro-tears in your muscle fibers or myofibrils. A torn myofibril activates satellite cells from nearby fibers, which rush to the damaged fiber and begin the rebuilding process. After your hill climb, it's good to give your sore muscles a good stretch and some time to rebuild. It'll take several weeks to a few months for noticeable growth through the rebuilding process.
Find Your Routine
Getting a good hill-climbing routine depends on where you live, but most areas have some significant elevation nearby for you to push yourself on. Look for a hill that is larger than what you're capable of riding up in one push, so that you can track your progress over time as you get closer and closer to the top. In cooler weather, you'll want to carry some warm layers in a backpack, including a wind jacket. Descending can be very cold, and it's possible to get very chilled if you work up a serious sweat on your way up.
Interval Boot Camp
Hill climbing is a good high-intensity interval exercise, where you push your leg muscles to their maximum and extend beyond your anaerobic threshold, a measure of the maximum amount of oxygen your cardiovascular system can transport. In an anaerobic state, your leg muscles will quickly fatigue, but over time your body will be able to keep up a high cadence for longer without tiring. If you're breathing heavily and you feel wiped out, you're giving your legs a great workout.