Here is a great workout I used to increase mental toughness and body endurance.  The workout was given to me by a friend who was a Navy Seal and uses only your body weight.  The workout is done in rounds and in order of the exercises with different movements going away after round three:

Calisthenics Program

Sets                Reps       Exercise

5                35                push-ups
4                40                bent leg crunches
3                30                dips
4                30                flutter kicks (3 count is 1)
3                15                hanging knee-ups (straight)
3                15                hanging knee-ups (obliques)
3                15                body builders (squat thrust with push-up at the bottom)
3                15                wave offs (on stomach doing a continuous superman and freestyle swimming motion, 3 count is 1)
3                20                lunge (each leg)
3                30                squat
4                15                standard pull-up
4                15                wide grip pull-up
4                15                commando pull-up

You should be doing a calisthenic workout at least 1 or 2 times a week.  If you cant do all the pull-ups start with a number of reps that you can handle and then add to it each week.  By the end of 5 weeks you should be able to do the whole program within 30 minutes.  Each week try to cut down your recovery time between each exercise.