And now, the guy with no professional credentials in workouts will embarrass himself.
Darren Barefoot linked to a semi-interesting but disappointing article about exercise myths.
Instead, here's my rough-sketched precepts of exercising, which draw on the Lifehacker article, but are more succinct, more coherent, and don't lie to you about the value of hard workouts.
Most people should do strength training. It is far more important than we previously realized*. Shorter, high-intensity workouts and drills are more useful than long-slow workouts; unless you're training for an endurance cardio event, you don't need a cardio focus in your workouts.
Go hard enough that you suffer, stop before you injure yourself. Fundamentally, exercise works by stressing your system, which causes your body to respond by building up the stressed systems. (Which is why you have to leave recovery time between workouts).
And now for some dark, disturbing news: you may be one of the people for whom exercise response is not very high. I don't really have any advice for you.
Choose a workout process based on what motivates you and what excuses you fall back on. In my experience, this is the most important element of your workout. If removing barriers to exercise means always ensuring your yoga bag is packed and near the front door, that's what you do. Get a workout buddy. If you're competitive, start a Big Loser challenge at your office. If you're too busy, do 10 push-ups every time you have a minute alone. If you like dogs, run your dog once a day. If you are cheap and motivated by deadlines, ride your bike to work.
That said, I don't follow this advice well. I don't do enough strength training yet, because I've been bad at finding my motivation or nudges. I do a ton of cardio, because I am training for endurance events (bike racing, where I need to make it to the end to unleash my sprint). Nonetheless, I'd encourage you to do push-ups, sit-ups, weights, and whatever it takes to build strength. The treadmill is largely a waste of time, except for distance runners.
*I'm feeling a little lazy, so no links to evidence for this, but the few studies I've read reports on and the general thrust of recent exercise thinking all point in one direction: strength is the core of fitness and ability, and strength-focused workouts are the core of building that. Cardio can be added on top of that base, if you need.