Ab exercises are TOUGH!

I just started watching my weight and exercising last week. So far so good. I figure I can lose a pound a week just by driving past McDonalds instead of stopping at every one I see. And the exercising is not as bad as I thought considering how out of shape I am. But the ab exercises are real killers! I did 5 crunches yesterday and started getting muscle spasms in my gut - no joke! I do a lot of other exercises without major difficulty.

Should I come back to crunches later when I'm in a little better shape, or tough it out?

Thanks!