I certainly won't be at it at 50, been at it since leaving High school, time for me to crash & burn at 50.

You might too just want add a little variety to your core/ab work just too mix things up a little, to the 600 crunches you do (Good exercise), upto you...

• Weighted Sit-Ups (Use Abdominal Board) (Recommended). (Weight plate behind the head not lying on the chest).
• Dumbell Side Bends (Recommended).
• Hanging/Lying Leg Raises (Recommended).
• Incline Leg Raises (from Abdominal Board).
• Russian Twists (Weight plate/Medicine ball).
• Incline Russian Twists (Weight plate/Medicine ball) from Abdominal Board.
• Lying/Sitting Russian Twists (Weight plate/Medicine ball).
• Incline Knee Ups (Ankle weights). (from Abdominal Board).
• Lying Cable Leg Raise
• Abdominal Wheel
• Medicine ball work (Recommended). Overhead throws/Chest throws/Lying or Sitting Pass Arounds/Lying Toe touches.

Some sport scientists believe core work will bring the biggest dividends in enhancing any athlete over ANY exercise. If your core muscle groups are not strong, you are not as fast, quick or mobile as you can be.