Congratulations on your new start! I think the best advice comes from the sage people at Nike "JUST DO IT!" Going to Curves, dancing around - it is all exercise and it is all doing your body good. If the sit ups are too painful right now then do something else. Believe it or not, once you get started on a good work out plan you will only want to keep improving it. At some point when you feel you need a good challenge, try the exercises you couldn't do in the past.
It is a slow progression but your body will continue to respond, strengthen and look great.