As I have said, general fitness is about five things: Endurance/Stamina, Strength, Flexibility and Balance, Power (Power = Strength X Speed) and a trim, energetic physique. Most exercises have overlap - addressing some aspects of all of the above to varying degrees.
It's possible to have good endurance (eg a distance runner) with poorly-developed strength and power, or to have excellent strength (eg a body-builder) without power or endurance. Perhaps the sport which demands the ultimate balance of the categories is high-level Basketball, but we can argue that all day.
"Functional fitness", as opposed to fitness targeted to a specific use or a specific sport, is aimed towards those four or five goals in a balanced way. For average people like me determined to "get in shape," the limiting factor to developing in all areas is strength (- if it's not being fat). Decent - not extreme - muscle strength is the foundation of everything else.
For example, it even requires strength-development to do effective, intense cardio intervals. Plodding along for hours offers no cardio or other physical benefit at all unless you are 80 years old.
That's why an introductory program to get back in shape will entail devoting plenty of your time with difficult and painful resistance exercises for the upper body, lower body, and core (back and abs). As you approach your personal strength goals (which ought to be modest and realistic because you ain't Arnold), you can back off to a more efficient strength-maintenance mode and focus more time on the other fitness areas - applying and stressing some of that strength by using it in more functional exercise like heavy calisthenics. Of course, that includes sports, recreation, and physical work and not just gym work.
Then, to stay in good shape, you'll have to do some of that forever - or until you give up on physical fitness and vigor and retire to the olde rockin' chair or the La-Z-Boy to decay in comfort.
That's the basic theory for maintaining vigor.
As a middle-aged guy who had not exercised seriously for 20 years, I think it will have taken me a year before reaching half-decent overall strength (eg benching my weight, deading well over my weight, good barbell squats, doing some number of chin-ups). As we get older, strength is more difficult to increase and quicker to disappear when it's not used. A sad fact: In middle age, muscle strength and mass diminishes measurably within 2 weeks unless it is stressed.
It has been fun to learn about exercise physiology. Physiology is interesting it itself, I think. ATP, fast-twitch and slow-twitch, aerobic and anaerobic, glycogen and fat-burning, muscle micro-tears, etc. Our animal body is a miracle.
A few interesting fitness links I've seen lately:
7 Ways to Boost Your Endurance and Stamina
What's the Best Way to Build Endurance?
Strength vs Power
Strength and Power: What's the difference?
7 Exercises to Gain Explosive Power
and some tips and comments:
- Avoid having a workout routine. Even with cardio. Reason is, with routine your body gets too efficient to be properly challenged. So if you run one day, do stairs the next cardio day. Or if you do squats one time, do jumping squats the next time. If you do bench one week, do inclined bench next week. Keep surprising your body with different demands or you will plateau.
- Free weights. Free weights are better than machines because they engage all the accessory muscles. Many machines are just to help people get to the point where they can use free weights without injury.
- Once per week is enough for heavy resistance work for a given muscle group. That's why it's common for people to have one upper body day and one lower body day per week. Or, in my case now, one heavy Push day and one heavy Pull day.
- Drop Sets are cool. By the end, you can hardly pick up a pencil or take a step. You are broken, shredded. That's what makes you stronger.
- It's truly hard. Strength-building under supervision is remarkably difficult mentally and physically. It might be the most difficult thing I have ever done, and the only immediate gratification in it is when the hour is up. That's because, as soon as you can do something, your demands are increased. Progress is slow, measurable in months, not days. It is no wonder that gym memberships and attendance peak after the New Year and dwindle thereafter. Without self-discipline and the right diet, it's a pointless pursuit.
- Technique is key. It's easy to injure a muscle using poor weight technique. You have to be taught. It's not as simple as it looks. (Even jump rope has a proper technique.)
- Make noise. Gotta let yourself grunt and groan. It helps a lot. And let those farts rip during deadlifts. Everybody else does, including the gals.
- Lazy, unfit, and overweight is a rational choice. It must be, because people are not stupid. Worldwide, it seems to suit many or most people very well. It is limiting, though. As I struggle with my program, I grow more sympathetic with their choice.
2015: 
Christmas Party at the South Danbury Church
From 
Chart via 
Camille Paglia 
Lettuce 
You're Right.