1. Aerobics or "cardio." Run, swim, bike, walk, hike, climb, skate, ski, etc. This is endurance training. It's good for "type I" (slow twitch) muscle fibres, which require only 24 hours to recover. Aerobic fitness involves breaking down stored fuel 16 times more efficiently in the presence of oxygen, and it confers endurance and long life. You have to move enough to breathe hard and deeply, and keep at it for an extended period, for substantial portions of an hour, or more. It charges you up on endorphins as an added benefit, and prevents or heals a host of inflammatory illnesses (though not "all" of them). .
3. Core/functional training. Here, we're talking about rotation around all of our joints, especially around the waist, off-balance movement and recovery (which improves balance), flexibility, functionality, fall and accident prevention, reaction time, stability, small-muscle strength, the ability to handle complex and unexpected movements, etc. While just pumping iron is fine for strength training (above), a totally different exercise strategy is needed for the twisting, bending, stretching and gyrations of (complex and integrated) core movement. The Pilates method emphasizes training the core of the body, and is a good starting point for exploring this type of training.
4. Lifestyle fitness. You can't just lock exercise into a scheduled time slot each day. You will STILL die years earlier than necessary. You have to incorporate exercise into your daily routines. There are 1-minute and 4-minute workouts you can do at almost any point throughout the day. You can park your car and walk, invent excuses to get out of your chair, take the stairs – not the elevator, walk for pleasure, change your position, get involved in physical recreational activities, etc. Fitness has to be part of your lifestyle, or you're not getting it!
5. Short-burst training. Also known as high intensity interval training, this is a new and hotly-researched area with some very convincing scientific evidence behind it. You go “all-out and whole-body” as much as possible in "bursting" movements (e.g., jumping, squatting, lunging, sprinting, throwing, kicking, punching and more) for intervals of 20–60 seconds (depending on the intensity level and the equipment/apparatus used for training) before entering the recovery phase, using a series of high-intensity, short-duration exercises interspersed with brief periods of lower-intensity movement. (Self-injury is not necessary to achieve the “short-burst” effect -- never push hard enough to hurt yourself!)
The intent of short-burst training is to utilize the anaerobic energy system. The primary fuel used is carbohydrate (which gets exhausted quickly), with stored fat kicking in later (in fact, for up to the next 24-48 hours -- this strategy is perhaps the ultimate "fat burner"). The conversion of white-fat "storage" cells to beige-fat "energy burning" cells is believed to be triggered by signalling molecules that muscles release during contraction (one of them is called irisin or FNDC5 -- though the function of this particular molecule remains a topic of scientific controversy at present).
The process of burning more oxygen for many hours after intense exercise is called "EPOC" (excess post-exercise oxygen consumption). A minute is the maximum "magic number" for a human to go “all-out,” and 20 seconds is sufficient when you are exercising continuously and varying your rate of exercise to achieve high intensity interval training. Research has shown that exercise intensity has a 13.3 times greater effect on systolic blood pressure, a 2.8 times greater effect on diastolic blood pressure, and a 4.7 times greater effect on waist circumference in men when compared to exercise duration.
In brief, to benefit by exercise, you need to incorporate all five of the above strategies in your exercise plan.
Here is the good news: Strength, core and short-burst training are EASILY combined, so we can beneficially treat these three as one group (by planning our workouts thoughtfully). Bursts are also easily incorporated in aerobic and lifestyle activities, so short-burst fitness can be fit into many places throughout the day. And yes, the research confirms... as few as three 20-60 second bursts of higher-intensity exercise within a workout session of only 4 to 30 minutes in total can have measurable long-term benefits. Similarly, on the other side of the coin, 15 minutes a day is sufficient to win the endurance benefits of aerobic (cardiovascular) exercise. So you ARE NOT wasting your time to be trying out workouts even of just a few minutes in length.
My conclusion: There aren't many excuses left. Being fit is TOO EASY!