Don't stretch well before you go on a long run as you'll increase your chances of injuring yourself. Instead, run at a slow pace for a while to warm up your muscles (like fifteen to twenty minutes) and then stretch, and then really go on your run.
Stretching 'cold' muscles followed by immediate prolonged high stress physical activity is a bad idea. Warming up your muscles is much more important than stretching. In recon training the best athletes out of all of us generally don't stretch at all during an event.
Anyway my advice is not medical so don't take it as such, it's just personal experience and observation.
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