General body warm-up (2-3 minutes) - add arm movements to your walking. Be sure to work the muscles of the chest, back and arms. The goal is to get the heart rate above 100 beats per minute or until you feel comfortably warm and you can tell your breathing is heavier than normal.
Pre-stretching and limbering (3-5 minutes) - gently stretch the major muscles of the arms, legs, chest and shoulders. This is a teasing sort of held stretching and not a major all-out stretch. Be sure to take the joints through a full range of motion during this time period if the warm-up did not include it.
Cardiovascular warm-up (3-5 minutes) - This is the time where you gradually raise the heart and breathing rate from warm-up to aerobic conditioning levels. This means to gradually increase the activity until you perceive you are exercising at 50-80% of capacity. The speed of the movement and the amount of resistance are gradually increased in order to produce this effect.
Aerobic conditioning - This phase should last 20 to 30 minutes of continuous, rhythmic activity going through as many exercises and as large of range of movement as possible.
Cardiovascular warm-down (3-5 minutes) - this gradually decreases the intensity of the activity until the body gets closer and closer to returning to resting state. The heart rate should drop to 110 beats per minute or less during this phase. You do not want the activity to stop suddenly and the blood to pool in the muscles. Gradually reducing the speed of motion and the resistance is the way to accomplish this.
Additional toning if desired
Stretching (5-10 minutes) - a good stretch here will last up to 24 hours. Stretching should be done in proportion to the workout. The harder the workout the longer the stretching. This is where the greater gains in flexibility can be reached. When stretching in the water you need to maintain body heat by moving the non-stretching body parts or alternating between stretching and a light exercise to keep the body warm.