As well as having control over achieving your goal, you need to be able to measure it too.
People will generally overestimate what they can achieve in the short term, but underestimate what they can achieve in the long term. So, having realistic short-term goals which reinforce improvement, within an overarching goal of something longer term and more grandiose, is key.
Keeping goals short also improves motivation. Rather than saying, ‘this is something that I'm doing for the rest of my life or the next 20 years’, you're breaking it down into shorter periods. In other words: ‘This is my goal for the next six to eight weeks.’
If you just started running, being able to run 5K might be an initial goal. And that can be broken down even more. The first step is to run 1K, the second step is 2K, the third step is 3K. Once you’ve worked up to 5K, you might set a time limit you want to do that 5K in. I recommend six to ten week periods, so you keep getting the reinforcement that you're improving and you're doing well.
Another example of a good exercise goal could be improving your resting heart rate – it’s something you can control, and is a sign that you're getting healthier. Timing your exercise or measuring the distance you're able to run or ride or swim are also easy, measurable goals you can control.