The amount of PURE CRAP in this thread is HILARIOUS. There's some truth in here but most is uneducated BS...seriously, if you don't know the answer why bother posting?
Here's truth:
OP: Have you been regularly working out? If not, then sure, go ahead and do pushups and situps, do them everyday if you want. You're going to see gains right off the bat simply because it's new to your body. However, soon after as you may be able to do more and more pushups and situps you'll eventually not SEE the type of gains that you're hoping for.
The military do pushups, situps, and other aerobic exercises all the time, however, and these will get someone in shape (due to their cardiovascular nature) and help gain stamina. They also aide in discipline.
So...go ahead and do your pushups and situps everyday. It's not going to hurt you at all, but don't expect miracles because they will not happen. A six-pack comes from diet, a strong-body comes from doing a variety of exercises, diet, and rest...