[vlad]

You almost nailed it. It's not how much exercise you do during the week, but that you spend time to do it regularly. Again, it doesn't matter exactly what you do. The point of exercise is that it challenges you a bit. So, if you go to the gym 4 times a week, even half hour before they close, you will be able to get 20-25 minutes of running in.

That consistency of showing up, even for 15 minutes a day 4 times a week, is what builds the habit. Building the habit is the important part. After you get the habit, then you will naturally start doing more challenging things to avoid boredom, because the constant is that you keep going, but what you do is what may change day to day. As well, you will find yourself setting aside more time to go to the gym because you'll feel more confident with your surroundings and the people. You may also become interested in classes, and start promising friends that "you'll see them at 5pm tomorrow." As you get (or stay) in good shape, you will learn about and be invited to various other athletic events or sports, which will reinforce the habit to stay in shape.