Ok pomu is running a marathon. So I will dump some running advice in the thread.
Your body has 2 energy systems, aerobic and anaerobic. This means "oxygen" and "without-oxygen". When you run at a controlled pace, you use the oxygen system, when you switch to a sprint, you go anaerobic and start producing lactic acid. That's why you can only sprint for short distances. If you're doing distance running, it's all about aerobic training. And that's possibly the most healthy thing for your body as well. You are working out your heart, getting a super powerful heart, that's the objective. As you train, other systems improve, like your entire cardiovascular system, your VO2 max, your muscles and ligaments....everything.
But one thing you must understand, being a powerful long-distance runner is not really about your muscular strength, it's about the power of your cardiovascular system. Your heart. If you're trying to buff up your legs to become a faster runner, you're doing it wrong. Although an important note, strength training CAN help you prevent injuries. The muscles to focus on are different for each person, depending on which injury they are more susceptible to. You need the help of a trainer to figure that out.
To become a strong runner, you must run every single day. We call this "putting on mileage", the higher your mileage per week, the better. Although higher mileage also puts you at risk for injury, it's about balance. Strong runners will do 60 miles per week, 80 miles per week....competitive marathon runners will even do 120 miles per week. Basically you should be running every day, and keeping it 100% aerobic. Do not do anaerobic training, unless you're shooting for a shorter distance race like the Mile. "Aerobic" doesn't just mean "slow", you can go pretty fast actually, especially once you get some training.
You should not be doing lots of miles on concrete. You will destroy your legs.
In terms of impact:
Concrete < Asphalt < Dirt < track < short grass
Avoid concrete as much as possible, if you can switch over to asphalt you are saving your legs.
Also you need real running shoes. Asics trainers, something like that. Do not do heavy miles in tennis shoes, you will destroy your legs.
You can also look up stretches and stuff, everyone has their own particular routine really. A coach helps a lot here.
Your running form is very important. This comes naturally to some, but others need coaching. Basically: chest up, hands lightly clasped with thumb over forefinger, you want your foot to be travelling in a circle. All of your energy should be directed forward, not up and down. In a distance race, don't propel yourself with your arms, they are just moving comfortably helping you balance.
Don't go out too fast in the race, the absolute worst mistake you can make. Keep it controlled. If you want to speed up, try it in the latter half of the race. It helps a LOT if you can find someone to break the wind for you. Run behind their shoulder, and match their pace. Then move up to the next person. Then the next.....etc. This is called "drafting" you save a TON of energy doing that.
I can't think of any more advice right now