What if your goal is to get stronger and to be able to workout longer, or just be able to hang longer with the kids when you take them sledding? How do you do it?
You use the overload principle. If you want to make a muscle stronger, you need to work it harder than it is used to working so it has to adapt. If you want to build your arm muscles, work those muscles hard enough that you fatigue them. Then your body will build stronger muscles because you need them.
This is a specific principle applied to weight training, but it can also apply to cardiovascular workouts too. If you want to be able to jog longer or harder than you can now, you have to push yourself harder than you think you can now. What happens when you do? One of the ways your body adapts is by building more blood vessels. When this happens, blood can get to your muscles faster and in larger quantities. This makes you be able to workout longer and harder with less effort. Your body is more efficient. This is one of the reasons working out helps you have more energy all during the day.