When "Hustling" and making a double move - move as a Move action, and a move as a Standard action - you may not make any attacks during a round in which you hustle (though you may still make AoOs).
The text in the movement section of the rules is for determining overland speeds. Somone with a speed of 30' who moves twice in a round has moved 60' in 6 seconds - they are moving at 10' / second, or roughly 6.8 miles / hour. This is that character's "Hustle speed." If, instead, they move 30' (a move action) and attack once (a standard action), they've moved 30' in 6 seconds, or roughly 3.4 miles / hour over the course of the round. However, should it ever become important, what they've really done is move at 6.8 mph for ~3 seconds, followed by something else for ~3 seconds.
EDIT:
To put it more simply, "hustling" means moving at a particular speed across the ground. For your average human, it means moving at ~6.8 miles / hour. If you do that for the entire round, you've used up all of your available actions (a move action and a standard action) and can move 2x your [game] speed (as you initially outlined). You don't get to attack. If you do something else during your round, you still move at ~6.8 miles / hour, but you only move for 1/2 the amount of time, and you therefore move 1/2 as far.
EDIT2:
Note that, in 3rd Ed, "Double Move" was a full-round action, and was equivalent to "Hustling."
SRD said:
Double move [Full][Varies]
Description: A character can move up to double base speed.
The space where a character begins the move is not considered threatened, and therefore enemies do not get attacks of opportunity for a character's move from that space.
SRD said:
Hustle: A hustle is a jog that is movement at about six miles per hour for an unencumbered human. The double move action represents a hustle.
I may be wrong, but I seem to recall the Double Move action actually being called the Hustle action in my 3.0 PHB.