What holds most people back is their fear of failure or conversely, their fear that everything will work out but that their life will not change.
It's important to recognise early on what your motivation is. Are you pushing for a goal for the wrong reasons? - like trying to lose weight to keep a man or achieve success so you can be flush with cash to buy boats and houses etc?
Many people who set a goal with a vision in mind that their life will change dramatically are often surprised by the result, this can be positive or negative. Either you lose the weight and you realise you're worth more than the ill treatment in your unhappy relationship or you lose the weight and absolutely nothing changes other than your dress size.
Similarly, those who make wealth their goal for the wrong reasons - ie expendable income, often find their funds depleted and themselves back to the drawing board again without a plan for their spending or income growth - much like Lotto winners who end up broke a year after winning the big one.
It's all very well and good to dream of the good life but if your vision isn't rooted in something of substance, without reference as to why you've chosen this goal, you aren't going to find worthwhile success.
If your goal is to lose weight, address why you want to slim down - is it:
a.) spice up a dying relationship?
b.) to grow your self confidence, or
c.) from a legitimate point of view of health and fitness?
If your answer is a.), weight loss will do nothing for your relationship, if anything you'll find you receive more hostility from your partner about the way people start to notice you more than usual or the reasons they imagine you're 'really doing it'.
If your answer is b.), weight loss may be a physical way to improve your self confidence around your appearance but if you're not confident in yourself, you may find these confidence issues actually amplify once you start to get comments on your weight loss. There are far better ways to increase self confidence on a deeper level wherein you're better able to find peace in yourself than putting stock in what you think others might think about you.
If your answer is c.), you're on the right track, your goal comes from a place of practicality, of the betterment of your health, in line with your goals to run that marathon or stave off heart disease. This is the kind of goal with longevity, one that is achievable, one that you won't beat yourself up for 'slip ups' and one where you can have some pay off in knowing what you've worked for.
Address your motivation for setting and achieving a goal and if it is found to be of substance, pursue it courageously. You can and will achieve your goals if you know the root of why you so desperately want to achieve them because fear has no place in the practical.
Be courageous - fortune favours the bold.