Many times I am asked how can you do this at your age.  How can you not is the real question?  To me staying fit is a LIFE STYLE not another job or hobby. I did read a book which basically stated:

  "You have no choice at the age of 50, you must exercise everyday.... period!!!!"

Rest is the Best:  Reviving your strained muscles and joints from any training is important.  As youths 5-6 days of training is normal but as we age the need to recouporate is essential.  Yes I can do the P90X routines daily because they alternate different body parts but my running workouts have 1 or 2 days of rest between workouts.  I never do the P90X Plyo on the same day of a track workout that is not considered rest for my lower body.  Structure your workouts carefully.

Weight Train: Muscle Burns More FAT ! So start very small with even simple body weight exercise.  Learn proper form using google searches. Wall Squats,
lunges, situps, pushups (start out on your knees or standing against the wall), etc. just to name a few.  Start small and gradually increase volume & reps to stimulate muscle strength.

Intro Weights as you Progress:  Start with light weight and increase slowly as you body can handle more weight thereby increase muscle strength.  Form, Form, Form is always the precaution.
Walk before you Run:  If you have never done much walking, then don't go out and expect to walk a mile, especially don't go out a run a mile either.  10% RULE !! Only do 10% more than what you did last week.  On day one, walk one block and come back home.  Do that for One week.  Add 10% for the next week.  That is not doing a half marathon.  Get It !!!   Too many people try to much too soon, get hurt, and go back to the couch.  This is no fun.  Please take it from me.  Take it SLOW.
Bike and Swim:  Again start small and work it slowly.  All of this eliminates injury.