for pushups/situps, do what my army recruiter said. Time yourself for two minutes and do as many pushups as you can in that time. Go all out. This will be called your "max." Then, every two hours, do half your max. Every week re-determine your max and adjust your bi-hourly routine accordingly. Same process for situps. It's not tiring and it works fukkin wonders.
 
-Ferric