Yes, but the effect will not be obvious after one month. Moreover, if you just run and do sit-ups, the effect is not obvious. If you want to practice abdominal muscles, you need to use special tools and equipment to practice abdominal muscles.

You don't have to do 100 sit-ups to practice. Everyone's exercise level is different. You can do 3-5 groups every day until you can't move, so you don't have to count them deliberately.