Cadence (stride rate) and Stride Length Explained
Firstly, don't even think about increasing stride length until you have mastered optimum stride rate, or cadence.
Cadence and stride length are linked to properly landing under your center of gravity. There is nothing in front of you that is going to help you go faster. Many runners overstride, and it's almost always caused by heel striking, which in turn is almost always caused by overstriding (incorrectly trying to increasing stride length). Your feet should be landing directly under your centre of gravity. This is your hips. In fact, if you aren't looking directly down, you should not be able to see your feet at all. The degree of knee lift is proportional to two things - how fast you are going, and degree of incline. If you are not running up a hill, and you are moving along at a comfortable pace, there should not be much knee lift at all. Lifting the knee is lifting a weight. Don't do it any more than is needed.
A difficult part of the leg movement cycle to master is when the foot should be hitting the ground in relation to where it is in its cycle of motion. This is directly related to turnover, or cadence. As I mentioned, many runners will overstride, making hard contact before the weight of the body can catch up and this results in a perpetual braking action. You do not want to be landing on your heels. This does not mean you should be landing up on your toes, like a sprinter. The heels should make contact with the ground, but only a split second after the forefoot. The forefoot (or ball of foot) should take the weight of your initial strike. To an onlooker, it would appear as if the entire foot was making contact at the same time, although the weight-bearing area is towards the front of the foot.
If you overstride and heel strike, the strike of the foot itself momentarily stops forward motion, which the runner must fight through over and over again! No one wants this. Also, heel strikers are landing ahead of their center of gravity. Although it's hard to master, try to already have the foot motion just barely on its way back just before it strikes. This is very subtle, but an extreme way of looking at it is imagining you are log-rolling. If you were trying to keep your balance while running in place on a log that is rolling in the water, you would certainly follow this principle! Another principle you would follow while trying not to fall in the water is a short stride rate, since there's a small area under you in which it is possible to place the foot. Remarkably, the log-rolling analogy applies here as well, since the place where the log would be is exactly under your centre of gravity.
There should be three steps taken per second. This is a turnover (cadence) of 180 steps per minute or 90 pairs of steps. Interestingly, this still applies at fairly slow jogging speed, up to fairly fast running (but not sprinting). If you watch elite runners as they tire in the final stages of a race, they increase stride rate, not length. New runners will do the exact opposite, lunging each leg forward. As I pointed out earlier, this does nothing but create a braking action each time contact is made, and disrupts the foot strike under the centre of gravity. Employ little, short and quick steps when you are at the end of your next race. Resist the urge to increase stride length!
So, once a runner has mastered the proper cadence of 180 individual steps or 90 pairs of steps a minute, and has very good form, what else can be done (aside from more training) to improve? Now the runner is ready to go back to what he/she avoided earlier, which is stride length.
Since stride rate generally should stay the same through a wide range of running speeds, at that point the only way to speed up is to increase stride length, which must be done without violating the rules of proper form, which dictate that everything happens behind the runner, and that overstriding is inefficient. When you see an elite Kenyan gracefully running sub-5 minute miles, he appears to be almost jogging. It seems impossible. This is because his stride length is tremendous compared to an average runner, yet he does this while keeping the same form as if he were running much slower. He will not overstride, as he knows this is futile. It takes a long time to learn how to increase stride length without slowing down the cadence and overstriding. Again, remember, everything happens behind the runner. All the power is not in lunging forward, but in pushing, or toeing off. As an efficient runner speeds up, his push off becomes more powerful, and another thing happens - the degree of knee bend when the leg is behind him increases, in many cases, dramatically. The secret here is similar to the degree of arm bend, and the weightlifting analogy that went with it. The lower leg is a weighted lever, and a runner lifts this lever thousands of times in a run. The higher the degree of bend after the push off when the leg is behind the runner, the less that the weight of the lower leg is stressing him over time. The higher bend also assists in continuing the explosive power of the push off.
So, if you are at the stage where you have perfect cadence and want to improve stride length, start getting used to a higher bend to the knee after pushing off. Point your bent knee downward as the foot raises high behind you. Don't make the common mistake of high knee lifts. Again, this is not sprinting. The action and power of a distance runner occurs behind him or her, and is far more hamstring-driven than the quad-powered, explosive but inefficient (high energy expenditure) sprinter's technique.