Mindfulness is a state of being in which you bring all of your senses and faculties into the present moment. No distractions no wandering monkey mind.
Being a mindful runner means that I am fully present in my body. Aware of the feel of skin sliding over bone, muscles flexing and relaxing, wind on my skin. There is no separation between mind and body. No duality. 
Sportsman of all disciplines speak of being in the zone. Time takes on a flexible quality. The world seems brighter and more coloured. There are many ways to reach a mindful state but for me I find it easiest when running and especially when trail running.