Meditation helps my running because it teaches me not to get attached to my thoughts. Instead of clinging to the negative thoughts I get at some stage in every race or training session – you know, why don't you stop, your body's not made for this, it hurts, what if you're injuring yourself – I choose to gently favour something else instead of the thoughts. Gently favour my breath or the sensation of my feet connecting with the ground. I take myself out of my thoughts and connect with my body. If my legs hurt then instead of concentrating on the thoughts of the pain I'll put my awareness in the body part itself, usually my hip flexors or my IT band in my leg. With awareness in the place that hurts I usually find that it doesn't hurt as much as my head thought it did. Without the negative thoughts in my head I start to feel good and can keep powering on. It's like a moving version of a mindfulness body scan or the cosmic body technique for those of you that know it.
But then running also helps my meditation. What I learn from maintaining high levels of physical work rate over several hours is that if I try to force anything to happen through mental strength or control then I quickly get stressed, tight in my running style, expend more energy than I need and go slower. (When I experience this I tell myself to relax and go faster. In relaxing I allow my body to do what it knows to do.) After a few minutes of this I realise that I just can't keep up that level of stress and tension for however long I have to run and I just surrender. Not surrender as in give up, but let go and trust my body, trust that the training I've done before will get me through, trust that if I let go everything will go a lot smoother. That's something I take into meditation. (Don't try to force it, don't try to make anything happen. Have an intention to hold the mantra.) I can't use my brain, my intellect to conquer meditation just like I can't run 42km with my mind. There comes a point in running and in meditation when I realise where I am and how I'm trying to think my way out of it again - my default behaviour. And at this point, I just shrug, smile and let go.