I used to do sit ups but have give up now lol. I have a great wee application for my Tablet called the spartacus workout. The logo is a red helmet . It works the core mainly and will do the abs without to much strain on them. It has a beginner,intermediate and advanced levels.
I am still at beginner lol
I love to run. Run at your own pace. Always start slow , even very slow, just faster than a walk to let your body adjust to the different pace without over exertion and your body will let you know when its ready for you to go faster. run a mile, walk a mile and run another mile. 3 miles is a pretty good workout.
I am training for a half marathon currently but have ran into issues of a wonky toe.
Good luck in your fitness building.
Start slow and build it up gradually.