Its exercise mate, if you're not putting the effort in then you wont get results and IMO real effort will result in some pain that you have to work through until your back is strong enough for you to do these exercises without it causing pain.
Don't over do it, and don't do it if its excruciating pain, but it will be uncomfortable to start with.
Sit ups, Crunches and leg raises will help tone stomach muscles, but then it all depends on how much fat you have over that area. If you can't get the results with the exercise then you may have to put some changes into what you eat from day to