There are two idioms here. "The key" means the secret to solving the problem. The problem is how to quit smoking when you are addicted to cigarettes. So the key or secret is perseverence...continuing to stop this habit without giving up. The "road to success" is the other idiom. You can travel down a road to failure, or a road to success. Perseverence, (or determination, or will-power) is the method in which you will succeed in quitting your smoking habit. This method is called a road to success.