John Mosby wrote a post on PT. I'd like to put out some standards. Last I checked these were the current doctrinal standards, if anyone knows of any updates please let me know.
Rangers, 49 perfect form pushups, 59 perfect form situps, 5 mile run in 40 minutes, 6 perfect chinups from a dead hang, 12 mile foot march with 50 pounds of kit in 3 hours or less. This is the Ranger School standard, just to get into the school so they know the training won't kill you.
Special Forces, 49 perfect form pushups, 59 perfect form situps, 2 mile run in under 14:54, pass a 50 meter swim test. This is the minimum for Special Forces Assessment and Selection (SFAS).
SEALs, 500 yard swim under 12 minutes, 42 perfect form pushups, 52 perfect form situps, 8 perfect form pullups from a dead hang, 1.5 mile run in 11:30.
These are the published minimum standards to attend the training for each of those elite forces. If you go and struggle to make the minimum, then you will more than likely fail to achieve the course standards. Those who have "been there and done that" will tell you honestly that to survive the training you need to blow these minimums away.
And if you think that these minimum standards all seem alike, well look back in history and see what Grandpa was doing http://artofmanliness.com/2011/09/12/are-you-as-fit-as-a-world-war-ii-gi/ And it seems that there is a pretty broad agreement that you need to meet a certain level of fitness to be effective in combat.
The other day I had to take another Army Physical Fitness Test (APFT) as a TRADOC requirement to graduate the course I'm in. The course I'm in has no physical component in it, but a requirement is a requirement. So I scored the minimum for the 17-21 male standards on pushups and situps, then hit their 70% for the run (14:40 two mile time). I'm significantly older than 21 so my "score" was higher than a 180.
I'm out of shape, and I can still easily pass the minimum standards. What I cannot do right now is max any single event. This is pretty normal, during a career, or even during your life, there are portions of it where you will not be in peak physical condition. However, not being in peak condition is no excuse for failing to meet minimum standards.
So how do you get back to peak physical condition? The old acronym, FITT, for Frequency, Intensity, Time, and Type is a good place to start.
You need to work out frequently. 4 days a week is the minimum to sustain a level of fitness, but you can exercise every day.
Your intensity level needs to vary. Running is great, but long slow distance runs aren't going to give you the "burst of speed and power" that you need for intense responses. Training solely for sprints isn't going to condition your aerobic systems for long endurance. Your sprints should be anaerobic, with that in mind. Everyone would like to be able to hump 120 pounds of kit 16 miles in 4 hours across country, but even the SAS don't train to that standard.
Time: As your intensity increases your time will decrease, but you need to work on increasing that minimum time. If you can sprint all out for 60 seconds, progress comes not only in covering more ground in that 60 seconds but pushing out the max time of effort to 90 seconds and beyond.
Type: Variety is the spice of life, but monotony brings home the bacon. Lunges, air squats, pushups, situps, chinups, are all plain boring exercises that you can do anywhere and nearly anytime. You don't need to do a novel exercise every time you work out, but you need to mix it up. Kettlebell drills are great, barbell and dumb bell exercises are fine.
Some of my favorite workouts? Heavy bag punch drills, if you can hit and kick a bag continuously for 45 minutes you are doing good. Stair/Hill sprints, someone once told me that you expend 7 times the energy for one foot vertical as opposed to one foot horizontal travel, I don't know exactly how true that is, but sprinting uphill or stairs is a great way to build anaerobic capacity. 2 mile sprint workout, go to a quarter mile track, sprint one lap, one lap, two laps, two laps, one lap, one lap, with minimum rest between laps.