Athletes are usually concerned with building speed and strength, and with this focus it is easy to ignore another important "S": stamina. Stamina keeps the spring in your step--helping you sustain energy over long stretches of time. The two quickest ways to increase your stamina are building strength and increasing mileage (walking, running or biking).
Run in the morning, if possible, at a slow pace so that you can maximize your mileage. Morning running will give you energy through your day and will also help you burn more calories through the day; helping you to increase your stamina. If you cannot run, then walking or jogging is a good alternative.
Strength train with weights if you have access to exercise equipment. This type of training is also best done in the morning if you have the time. If you do not have access to weights, a good alternative is to do push-ups, sit-ups and pull-ups.
Do speed work on a track or run hills to both increase your lung capacity and build strength in your leg muscles. These types of workouts are helpful because they are like a strength and cardio workout combined.
Jump rope as a cross-training exercise to increase the amount of air taken in by your lungs. This is one of the most effective exercises for lowering your resting pulse and thereby increasing your stamina.
Bike on the roads or the mountains to increase your stamina. Biking increases stamina by allowing you to work your heart and lungs for a longer, sustained amount of time than running. It is also a great way to build strong, lean legs.
Start a daily yoga routine. Yoga can be a productive stamina building workout. It helps build long, lean muscles, increases lung capacity and fills you with energy.
Eat healthy, energy packed foods. In general these are foods that are low in fat, mostly unprocessed, high in protein, and full of vitamins and minerals. Focus on fruits and vegetables, lean meats and whole grains.