Walking meditation is an excellent introduction to mindful running.
Generally, the best approach is not to think of things that you can't easily change, such as your breathing or physical condition. If you focus on a mechanical running movement, or even on a still more automatic process such as breathing, this can be counterproductive (this is known as the 'constrained action hypothesis'). With mindfulness meditation, you instead concentrate on your overall sensations, or think about the things that normally come and go in your mind.
Astudy has even demonstrated that oxygen consumption in runners increases when they think of their breathing or physical condition (in comparison to thinking about the sensations they are experiencing, what might happen during their run or the things passing through their minds).
But an internal focus of attention "can provoke negative thoughts, particularly when the sensations become unpleasant over the duration of the run or as intensity increases". In this case, it is better to rely on your mindfulness training so that you aren't distracted by these thoughts – or for non-meditators, to visualise the party after the race or competition ;-)