I am a ultramarathon runner that has been actively running for around 3 years.
How do you know when you've gone too far? In the last several years I have been running a pretty good deal. I've done many races from 5k to ultramarathons, my most recent being a 33 mile trail race in the North Georgia Mtns, during this time I have had friends that are just getting into running ask for my advice. The most common question that I hear is, how much is too much? This is a pretty good one because no one wants to get hurt! Running too much can definitely have some negative implications for your knees and even mood when you begin racking up substantial mileage. There are a couple warning factors that will let you know that you are either running too much or not getting good rest.
First is your knees, if your knees are giving you problems its not the running its self but the mileage or place you are running. Try to run on soft dirt or trails to minimize this problem, if you have knee pain there take a break you've pushed too far.
Second is your mood, I notice that when I start putting in more mileage than can be sustained my mood gets worse as the miles drag on. This continues even when I'm not running as well, at work, home, and with friends I get more frustrated and grumpy. This is a good sign you need a break.
Third is pain, any avid runner should know that a little pain comes with the game, but anytime you get a sharp shooting pain it's time to stop and recover. Even if you're training for a race or event just know that running when it hurts is not going to help you. If or when you get hurt you will miss that race you're pushing so hard to go to, trust me I know.
Those are the three most common signs that I see that tell me I'm at my limit or near injury. Running is a great sport and a fun one that will keep you in shape, but remember friends everything in moderation! Even running! Thanks for reading and good luck on your endeavours.