Most Commercial games use WASD keys for player movement, and when you create your GC Game, you should not change this. Players are interested in playing your game and not in learning new game controls and this has been shown in extensive research.
With this in mind, holding down the Left Shift key is the common way to make your player character go from walk to run. This short tutorial will show you one simple way, not the only way, to acheive this.
You will create two triggers, each with one Action. You can name them anything you want, but I have named them as shown.
Set the First Trigger from the Trigger/Input menu to On Key Down, and select Left Shift.
Add an Action and add Can Run (selected) from the Character/Character Properties/Can Run menu
Create a Second Trigger from the Trigger/Input menu to On Key Up, and select Left Shift.
Create another Action and add Can Run (unselected) from the Character/Character Properties/Can Run menu
You should also unselect the Can Run check box for the Player Character, so that Walk is the default.
And thats it!
Of course, you can also achieve this with Variables, using the Toggle Bool is the most efficient, but we will cover that next time.
Happy Game Making.