Strength and stamina are built by stressing the body for an achievable time interval. Interval training is one of the best ways to increase speed. Warm up easy, run at the limit, then cool down easy.
By carefully monitoring and enduring stress we strengthen ourselves. Running at an easy pace builds endurance. Running at our limit builds strength and speed. Combining easy runs with interval runs we learn to run farther faster.
Carefully monitored and controlled stress strengthens us.