Muscles used in sit ups vs. crunches
I think what happens when you do sit-ups is your hip flexor muscles are actually the prime movers here and the abdominals are the stabilizer muscles keeping your trunk straight.
In crunches you focus only on the rectus abdominis and your obliques.
Check out these sites.
http://sportsmedicine.about.com/cs/conditioning/a/aa020402a.htm
http://www.exrx.net/Lists/ExList/WaistWt.html#anchor172895
Hope this helps a little.
Jim
Try some jogging, or
energetic walking to supplement your exercises and workouts. Any seasoned runner knows that the abs and back benefits, and contributes to the running process. The aerobic activity should contribute also to the longevity and enhance the quality of your paddling career. Good luck,
mickjetblue