A review article in healthnews covered the typical health benefits that have been seen for everyone who practices mindfulness. It also discussed ways in which it specifically contributes to athletic performance. There are good references to some scientific studies supporting this:
Current Sports Medicine Reports. Mindfulness in Athletes.
Benefits for Athletes:
NIH News In Health. Mindfulness for Your Health.
I know from personal experience how it helps performance in endurance sports. People deal with the discomfort of performing endurance activities in two ways: associative and dissociative thinking (see here for a review of this for running). Associative thinking is about the activity itself. Dissociative is thinking about something else to distract you from the activity. Research has shown that associative leads to better performance. That has been my personal experience. I can go faster if I actually keep my mind on the activity rather than distract myself from it. Mindfulness fits in with this because it is associative, the idea is to be fully present with the activity. I notice the rhythm of my breathing, and how it synchronizes with my pedaling stroke (or footsteps while walking). I find this is relaxing, even if the pace is challenging. The mind will come up with complaints like “This is uncomfortable”, “why do I have to go for so long today?”, etc. I don’t fight these, but let them go and return to breathing. So that makes it meditation-in-motion. This can also help lead to the “flow state” mentioned in the healthnews article which makes the activity very enjoyable.
Of course, if you enjoy your current activity and like to dissociate while doing it, say by daydreaming about your upcoming vacation, there is nothing wrong with that. It probably won’t lead to as fast performance, but that may not matter if you’re exercising for health reasons.