Skytzo_Marc:The human body was made for walking and running so naturally IMO, the best exercise is sprinting and jogging. You'll be surprised by how quickly you can build up if you rest properlly and push yourself.
I'd say that the idea that the human body was made for walking and running is debatable - it is certain that it was not "made for" walking and running on hard surfaces. As you age both you and your joints will appreciate it (last longer) if you try to minimize the pounding your joints (feet, ankles, knees, hips and spine) take. As an old (43) lady who used to do all of the joint pounding exercise that I could (years of competitive hard court tennis, rugby and running) I sure wish that I had been a swimmer instead. I swim now and think that it has the double benefit of increasing your in-water comfort level along with increasing your cardiovascular fitness. I have a pretty low boredom tolerance and hated lap swimming until I got my Ipod safely rigged for water and started listening to audio books while swimming. Yoga can also help you develop better breath control and nothing beats more diving for decreasing your SAC.
Jackie