Functional Fitness

I subscribe to the philosophies of fitness described by Matt Furey (mattfurey.com)
More than just his combat conditioning exercises, I like the definition and goal of functional fitness as he describes it "Do you have the speed, strength, agility and endurance to get yourself out of a life threatening situation?" Can you swim in your clothes, climb a steep slope, climb a rope, run to catch something or get away? Basically, do you have the strength to move your own body where you need it to go?

This naturally meshes well with his exercise philsophy of calisthenics: exercises where the only resistance comes from your own body weight.
His three key exercises, or "Royal Court" as he calls them, are a great minimum if they are the only "exercise" you do. They work well as they build strength and flexibility through your whole body and particularly the core. Done with the right breathing (the only way you'll get enough endurance to do lots of them) it also gives a good cardio work-out.
The three exercises are:
Hindu Squats, Hindu Push-ups and back-bridge. He sells this for a living so go buy the instructional book or DVD (DVD is better) or ask me to show you one-on-one. Apparently it's very much like "Farmer Burns" as well.

I've found I don't have a lot of time or motivation for extended training at the end of the day so after trial and error I found that I can reliably motivate myself to do a short set every morning when I first get up. It also has the added advantage of waking me up and making me feel relaxed and energised to face the day (gotta love those endorphins).
Rather than trying to do lots, I focus on just reliably doing it. Each morning I set a goal of how many I feel I can do. Most mornings I just do 20-20-20 (20 breaths of squats, pushups and bridge). If I feel a bit off colour, I set my goal lower, but I always make myself hit my goal unless I fall on my face or pass out.

I recently worked out that doing a single set leads to tightening and cramping of muscles after the exercise. So I now do two sets of 15 push-ups and squats instead of the original one set of 20.

For SCA combat I found that I wanted a bit more oblique abdominal workout so I've incorporated a set of 20 knee-to-elbow crunches. I also find this quicker breathing exercise really shakes the cobwebs out and picks the heart rate up.

It's all about the breathing. If you ever find yourself unable to do one or all of the exercises, sit or stand somewhere and breathe as if you were doing them. You can maintain or even improve on poor fitness just by doing focussed breathing.