By David A. Cooper:-

If you can run, you know how to run already, but how can you get faster? Should you just spend more time running? Well, it isn't necessarily that simple. If you like distance running, then you will probably improve by doing more running, but you may find that you only get better at running further without becoming a lot faster, and if you are a natural sprinter, running long distances is likely to make you slower when you sprint. To run, you use muscles in your legs, but these muscles contain two different kinds of muscle fibres within them: fast-twitch (for sprinting) and slow-twitch (for long distances running). Sprinters want to have as many fast-twitch fibres in their muscles as possible, while long-distance runners want lots of slow-twitch fibres, though they also need enough of the faster type to give them a reasonably strong sprint at the end of a long race. The kinds of fibres you have in your muscles are affected by the way you train, so lots of sprinting will make you grow more fast-twitch fibre, and lots of long distance running will create more slow-twitch fibre, but it does this mainly by turning fast-twitch fibre into slow-twitch. Sprinting cannot turn slow-twitch fibre into fast-twitch, so slow-twitch fibre is hard to get rid of if you don't want it: that is why top sprinters avoid distance running like the plague.
indentSo, if you want to be a sprinter, you should do lots of explosive sprinting and avoid doing any serious distance running, although gentle jogging over long distances is unlikely to harm your speed. You will also want to do some kind of endurance exercising to keep your heart in good condition, but you don't want to do too much of this using your legs, so your best bet is to do a lot of training based on boxing or swimming. The best sprinters usually have massive arm muscles, so my guess is that the way you move your arms while sprinting is important for helping you put the power down into the ground, and the more powerfully you move them, the faster you will be able to run.
indentIf distance running is more your thing, then you need to do a mixture of sprinting and distance running: the sprinting is necessary to create the fast-twitch muscle fibres that will be turned into slow-twitch fibres when you do your distance runs. One day a week, you should concentrate on sprinting: do five or six flat-out sprints over fifty to a hundred metres (depending on your age), taking five minutes or so to recover between the sprints. Never do this more than once a week unless you have a proper coach training you: it can damage your muscles and even your bones if you do it too often. Pick two other days of the week for your longer runs, but avoid training two days in a row because you'll need rest days to recover fully from your training.
indentImportant warning: running downhill is bad for your knees and other joints, so always ease off when the road slopes down steeply. If other people running with you don't do this, just let them go on ahead: they'll pay for it later in life, and they'll probably pay for it later in the run too, so just let them move away and chase them down when the road flattens out again. If you do lots of distance running, it's also best if you do most of it on soft ground rather than on hard roads and pavements. Concrete is the worst possible surface to run on, so avoid it completely: it has significantly less give to it than tarmac.

How fast should you aim to be at different ages?

I don't know yet, but when I find out I'll put all the information here. It will be a list of times and distances taking into account age, size and gender (boy/girl). There will be record times (so that you can see how you compare with the best), slower personal-best times from people who went on to be top runners later in life (so that you can see if you have a realistic chance of getting to the top), recommended target times (to give you an idea of how fit you ought to be), and slowest acceptable times (so that you can see if you're seriously unfit).