I see what you're asking. Even if you're getting in the situps throughout the day, an actual workout is still a good idea. Try varying the situps day-to-day with something like pushups or body weight squats.
Do you mean.. if I were doing for 3 days (100 situps a day), then for 3 days (100 pushups a day) etc. Cause I had that in mind.. but I don't think that will make me more musceled up.. because that is my goal.