Take the first step to something that scares the crap out of you.
Do it. Today.
Go sign up for a 5k, or triathlon if that is what you have always dreamed of. Contact a personal trainer, or weightlifting coach or fitness competition rep if those are your goals.
Life is far too short to sit around *thinking* about your dream. Go out there and chase them down. Step out of your comfort zone and set yourself up to *have* to do it.
The very first step is to get started, once you do that you will find the motivation and momentum to carry you through.