The problem with some affirmation techniques (I am strong, I am fast, I am fit etc) and with encouragement along the lines of "You can do it" or "You've got this", is that none of it makes any difference if you don't remember it (or believe it) when the going gets tough. In a mountain storm, or the last mile of a marathon when the seconds are ticking away -- hope is a poor substitute for knowledge-based confidence.
In countless conversations with elite athletes, adventurers and with clients who accomplish big goals -- a common trait which makes the difference between success or failure -- is a calm, impenetrable knowledge that we have what it takes and that we are just going to do it!
One way to develop this confidence is to visualize the results you expect to achieve, but write it in the third person, rather than the first person. This builds some separation between you and the moment you're writing about. Rather than wondering about how you'll do, the exercise has you step back and explain how you'll do it.
When you get on the race course, the trail, the mountain, or embrace a new fitness lifestyle, there can be some deja-vu and you can perform with the feeling that you've already done it. Moreover, you change your training because knowing everything that was needed, you were better able to design the perfect plan.
You write your own story.