WaterRabbit
Explorer
Not an existing rule, it would be a house rule.
As I said, this is mostly because I'm going to have athletics checks for increasing speed, and I need a point to build that on. Either I allow double dashing (action dash, bonus dash, move) and base "fastest human" around someone who can bonus dash who has the mobile feat ... Or I say you can't use dash twice and make those abilities about multitasking.
I'm inclined to disallow double dashing because people can jog at around 6 mph for hours, and that requires the dash action. Double dashing would require an extra endurance consideration in the rules I'm writing, and it's far easier to avoid it.
Action Surge is an interesting case. I'd be inclined to not worry about it since it is a per rest ability and not something that can be done every round like cunning action.
I think you are way too conservative on what people can do. People with long legs or a fast stride can walk at 6 mph. I walk at 5 mph and one of my buddies at 6'6" easily out paced me. People run at 10 mph jogs easily for a few hours.