Abdominal tone will not return on its own; it has to be built up with exercise.
Tummy tucks do of course tighten the fascia, or deep layer. However all this does is reduce the abdominal bulge. It does not strengthen the abdominal muscles. That is up to the patient.
Rather than just do situps, it is best to consult with a competent physical trainer on how to build up the core. Usually the surgeon will the go ahead 6 to 8 weeks postoperatively.
most surgeons answering these posts are fine surgeons and this issue should not come up, but make sure your intended procedure will involve tightening of the full fascia all the way to the breast bone. I have seen many patients over the years who did not have their fascias tightened during a tummy tuck, a shame since the surgeon has the fascia open and could easily have tightened it, and now the patient has a persistent bulge.
Also there are full tightening tummy tucks now that can be performed through low or minimal incisions in the right patient.