Do you grow anxious when pushing yourself to run fast over a mile or two? Does the effort make you feel as if your airways are constricting, that you can't catch a breath? Does your rapid heart rate seem to be accelerating out of control? If so, hopefully you've sought medical advice and been checked for any arrhythmias, asthma or other physical conditions. Ruling those out, might the problem be that your brain hasn't yet learned to cope with the physiological changes of a body labouring under effort?
I don't know why it's taken me so long, but I've only recently become aware of how distressed some people become when they try to push themselves hard over a few miles. These are people who can run very well over 5 - 13 miles, but who seem truly panicked at middle distances where hard, uncomfortable effort is sustained for many minutes. The feelings they describe are very similar to those psychologists consider diagnostic of panic attacks.
If you run purely for health benefits, mental and/or physical, then you don't need to run/race over the distances and at the effort that trigger such sensations. However, you might wish to take control of them and, thankfully, cognitive behaviour therapy (CBT) has established success in helping people deal with such distress. The core approach is to be mindful - curiously aware of sensations and thoughts, stepping slightly out-with them to observe them, marvel at them and then switch from one sensation or thought to another. Such mindfulness takes practice. Begin with something pleasant, maybe a small square of dark chocolate. Take it into your mouth, become aware of how it feels resting on your tongue, the shape of it, how the shape begins to change as it melts, the spread of flavour across your tongue etc. (In writing this I had to go off to find chocolate.) Be attentive to each sensation and gently try to keep your mind focused upon each sensation in turn, directing it away from the noises around you and the other thoughts that creep in. Try being mindful on a steady run, maybe focusing upon the smell of wet earth after rain, then the roll of pressure across your foot as it contacts the ground. Enjoy wondering about these sensations, enjoy being able to shift your focus and take control of your thoughts. Once you've developed your skills in being mindful, then you're ready to incorporate a bit of stress to be marvelled at and observed. Attend to the accelerated heart rate, marvel at how your body adjusts to supply oxygen to muscles, feel the heat rising to your skin, radiating outward. Attend and notice that you aren't burning, your heart is continuing to beat, your body is an amazing, efficient machine.
I use mindful running to feel connected to nature and to switch out of cycles of negative rumination. When I've got myself into a negative mind frame, 30 - 40 mins running outside is the best therapy. To feel really good, I'd add the hard exertion of speed intervals or running uphill or through mud/sand/water. The extra effort let's me be mindful of the strength in my scrawny body and that, whatever else is happening, I have strength.
"Exercise for Mood and Anxiety" by Michael Otto & Jasper AJ Smits. This is a wonderful book, written by academic specialists in exercise and motivation. They also seem to be as human as the rest of us, making their advice pertinent and easy to read.
"Mindfulness: a practical guide to finding peace in a frantic world" by Prof Mark Williams. This book and cd combination is very strongly recommended.