Rather than building muscle mass, they just tend to become stronger with resistance training. Olympic runner Florence Griffith-Joyner is a perfect example of this body type. Finally, mesomorphs tend to be muscular and strength training generally builds their muscle mass much faster than it does for their ectomorph sisters. As an example, picture Olympic gymnast Mary Lou Retton with her powerful physique. Work with slightly heavier weights and fewer repetitions.
Endurance comes from using lighter weights and increasing repetitions.
Use too little weight and nothing will happen. After several months of resistance training, most people will experience a 10 to 30 percent increase in muscular strength. The key is understanding your body type and how it responds to exercise. Avoid comparing yourself to others. If you focus on how good the exercise makes you feel rather than how you look, you will learn to accept your body as it is.