Strength & Endurance Training go hand in hand and when exercising, by increasing one aspect of your training (i.e. strength through weight training) you will improve the other (endurance). In time your body will adapt to the extra physical demand and in turn increase your muscle mass.
With limited strength training your muscles will reduce in size (muscle atrophy) so it is very important to keep them strong and flexible to support your skeletal frame and to prevent injury. Muscle atrophy will result in your muscles becoming smaller and weaker.
Depending on your physical demands you should specify your training sessions by performing Muscular Strength & Endurance exercises in some form for at least 20 – 30 minutes 2 – 3 times per week