Just about any movement is good movement from the point of view of your body, and, generally speaking, the more intense the better, within reason. The best technique for not skipping workouts is to make it a habit; do it at the same time every day so you don't have to think about it too much; after you have established a routine both your mind and body will be prepared for exertion at the appointed time.
The hardest thing you will do is sometimes just to get yourself to the gym, but once you're there and you get started you may be amazed to find that you can complete your workout. As they sometimes say in yoga, the hardest move is the move onto your yoga mat.
If I'm really not in the mood to exercise, I tell myself I'll just get to the gym and do one set; if, at that point, I don't feel like I can do any more, I'll leave without guilt. But once I'm at the gym and do the one set, I always find I can do more. One of the things you'll discover is that movement may consume energy in some sense, but it also makes you feel more energetic; movement spurs more movement. Even after a demanding session, I'll usually feel less tired than when I started the workout. As Newton's First law of motion tells us, an object in motion tends to stay in motion, and an object at rest tends to remain at rest. You want to be the object in motion.
You really should exercise every day if possible; perhaps with one day a week off. Most of us have very sedentary jobs, and don't get much exercise in the course of work; even jobs like construction have been mostly automated.
A little bit of exercise every day beats trying to make a heroic push on the weekend, which tends to lead to injury. Government recommendations aside, there is no set amount of time that is really required; the amount of time depends to a large degree on your intensity level. If you can run a mile in seven minutes you can get a good cardio workout in seven minutes. 15 minutes in the pool can be quite a workout. If a leisurely jog or bike ride is more your style, an hour may be required.
Weight training generally requires longer workouts even at high intensity, as some rest is required between sets. Even if you are training different body parts on different days, there's probably not much point in a weight training session if you can't devote at least a half hour to each session, excluding warm up time. Conversely, even if you're a professional athlete, you should probably not do weight training at a high intensity level for more than an hour.
While exercise should become a habit, that doesn't mean that you should mindlessly be doing the same things over and over again. The AM system is about an active mind governing an active body; you need to think about what you eat, and about your exercise routine. The most common mistake that most people make is simply not exercising enough, but among people who do regularly exercise, the most common mistake is that they don't have enough variety. Runners run, and do the same repetitive movement over and over again, often leading to injury. Lifting weights is generally an excellent way to get in shape, in large part because it provides the opportunity for a huge variety of different kinds of exercise. But lifters tend to lift to the exclusion of other kinds of exercise, and they tend to do the same lifts over and over again, and focus on some parts of the body; chest and arms, to the exclusion of lower back and legs.
The best workout regimes will provide for a lot of variety, and work all the parts of the body in different ways. One day you might do medium reps of bench press, dips, and military press for strength, focusing on shoulders, triceps and chest, followed by a light run for legs and cardiovascular fitness. Another day you might take a yoga class for flexibility; a third day you might do sprints up hills for the fast twitch muscles in your legs, as well as providing an intense aerobic workout. If you have access to a pool, there is no better exercise than swimming, but even here it helps to vary the workout by strokes and by distance. We go into more detail on different exercises below, but the key point here is that you should mix it up.
The human body is an extremely adaptive mechanism, and it must be constantly challenged to improve. A runner might think the way to do this is to run further and further; a weight lifter might think the key is to add ever more sets. But there is a limit to how long one can, or should, exercise; we all have other things to do in life, and the real way to improve is through variety and intensity, not by making workout sessions longer.