To turn on auto run in Fortnite Chapter 3 Season 1, go to Settings. You can do this from the main menu, or when you’re in the middle of a match. From the main menu, bring up the sidebar and select the icon that looks like a menu overlapping a cog. Then select settings and go to the Game tab, the icon for which is also a cog. Go down to Movement and switch Sprint By Default to on.
You’ll now run automatically whenever you move, and can use the Sprint button to slow yourself to a walk. Sprinting by default is very useful if you want to make full use of Chapter 3’s new sliding action, as you need to be sprinting in order to slide. When Sprint By Default is on, you’ll also automatically swim as fast as possible whenever you move in deep water.
Most players prefer playing with Sprint By Default on because in most situations it’s best to move as fast as possible, and if you do want to slow down, you can just crouch, which is quieter than walking anyway. But if you do decide that you’d rather run manually, you can just go to the same settings menu and switch Sprint By Default off in the same way you switched it on.