The day after intense exercise, bleeding and microscopic tearing can be seen in muscle fibers. That's why muscles feel sore the day after an adequate workout. Exercisers should allow about 48 hours for muscles to recover after exercise. When the muscles heal, they are stronger. Exercising two or three times a week, alternating exercise days with rest days, can help prevent injuries.
Fitness is determined more by the intensity of exercise than the duration.
Workouts should be hard enough that the muscles are somewhat sore the next day but fully recovered the day after that. The harder a person exercises, the faster the heart beats and the stronger the heart muscle becomes. Heart rate is determined by how hard the skeletal muscles contract. When a person starts to exercise, the skeletal muscles contract and squeeze the veins near them, forcing blood toward the heart. When the skeletal muscles relax, these veins fill with blood.