Is it safe to take aspirin before running a 10k

Actually, the body is response to pain often causes more damage than the cause of the pain. Reducing inflammation and swelling is a good thing.

I would not reccomend it. If your leg is swelling up, asprin is not going to fix the damage you might do to your leg by pushing it is limits; it will only make it so that you do not feel the pain, which is not necessarily good because pain is basically a warning that your body has to prevent further injury.

it thins the blood but do not know what the side effects could be for running.

its probably a good idea it thins the blood but thats a question you should ask a doctor or call the emergency room because every one is different and getting medical advise on line is pretty risky