The short answer? Keep doing it! The idea of these programs are to get you to a point where you can do all these push ups and sit ups. Same with the Couch-to-5k/10k programs. Building up your stamina and your strength sensibly is only part of the battle. The remainder is to keep it up.
Now, I don't know about you, but the idea of doing a bunch of push ups and sit ups doesn't sound like it will keep my interest for very long. So an alternative to keeping up with the existing program is to transition to another type of program that will challenge you further. If your goal is strength, you can only get so far with just your body weight. You might consider transitioning to weight training, or progressively adding weight to your body (such as the barbell plates on your back).
Alternatively, you can also pursue another area of fitness. If you ever fancied running, martial arts, biking, or swimming, those are all different challenges to keep you going.