When you run super fast like flash you need to be in a very specific state. Most people who are sprinting and training hard to increase their speed will tell you that they have to psych themselves up to get in this special state. This is also the reason why many of them don’t actually reach this speed – it takes too much mental effort to maintain.
The best way to learn to run super fast is by practicing running at high speeds and doing intervals. Try doing 60/120s where you sprint for 60 seconds and then slow jog (or walk) for 120 seconds to give you a good work to rest ratio. This technique can be used indoors or outdoors and you can adjust it to any level of fitness or ability.
As you run faster, each step becomes more and more powerful. This will make you airborne for longer, and can even allow you to cover large distances before gravity will overcome the force of your body weight. For example, the Flash can travel as far as 660 feet before he needs to take another step.
The other thing about super-speed is that it changes how you perceive time. This is probably why they have to “shift gears” – it would be unbearable to see the world around you freeze up as you move beyond the Speed of Light. Thankfully, most of them can only go up to Cruising speed before this happens.