Sex is Exercise and should be treated that way.

Tantric and karma sutra practices can help here.  

            Sex is not only a great exercise for your relationship but for your health also.  We all know a great sex life helps in building a wonderful romantic relationship but you should also treat it like an exercise.  Set time amounts for your sex.  Have 30 minutes of foreplay before you every touch genitals together.  Try different positions to extend the time you are actually making love.  Brush up on oral techniques and other less talked about sexual stimulations,   Just like when you work out to help your body you need variety and to push yourself from time to time.   Keep track of numbers and try to improve them until you both are comfortable with the amount.   Heck make a sexercise chart for the two of you to keep track of after each session of making love.   If you both put effort into this project you will both be rewarded so dust off that booty and get ready.