Faster moving on key press

Hello i searched very much to find a question like this but i didnt find so i posted one.
The question is: When i press A on my keyboard i accelerate my first person controller.

Thanks.

in Update function:

if(Input.GetKey(KeyCode.A)) {
    //change your speed variable over time, to fast mode
} else {
    //change your speed variable over time, to slow mode
}