You are either too fatigued or to weak and need to rest to recover or stop altogether. Start small and work up. My suggestion is to start small and work your way up to a more demanding routine. Also, many times you see people doing five hundred million sit ups. Know that if they are doing that many, they aren't doing them correctly.
Abs are just like any other muscle group.
If you do the exercise correctly, then you shouldn't need to spend a lot of time to get to the point of fatigue. Keep it short, intense, and use correct technique and you should need only 10-15 minutes max to get a good ab work out done. It will help improve on field performance, but it will not take the place of training with the ball and playing on a regular basis. Always, always check with your doctor before starting a new work our regimen. This is something that has contributed to my over all performance, as well as, longevity in career, however, it may not be the best or safest routine for you.