It is not a bad thing to do situps, but avoid doing them the wrong way because you can cause a lot of damage to your lower back. Try not to do them while you have your feet anchored. A good alternative to situps is to do crunches with a rolled towel placed at your lower back.
If you plan on exercising longer than an hour you need to consume between 45 and 60 grams of carbohydrates per hour of exercise. This will help your body work more efficiently, as your body can only store so many carbohydrates. Once these carbohydrates have been used, you will feel physically and mentally exhausted.
When working with heavy weights over your own body weight, you should always try and wear a weight belt. This helps keep your spine in line and in case something happens, it can prevent death or serious injury. This is essential with working out with weights that you might not be able to handle.