From the SRD fly description:
'Using the fly spell requires as much concentration as walking, so the subject can attack or cast spells normally. The subject of a fly spell can charge but not run, and it cannot carry aloft more weight than its maximum load (see page 142), plus any armor it wears.'
Fly and expeditious retreat don't stack. When you have fly cast on you, you use the spell to move, as opposed to moving your body. Just like when you drive a car, you use the car to move yourself, you don't actually move your body. That is why fly has a limitation on what type of movement it allows (cannot run). Expeditious retreat on the other hand is the equivalent of a booster shot full of adrenaline. It doesn't matter how much adrenline you've got in your body, it won't make the car your driving move any faster.
'Using the fly spell requires as much concentration as walking, so the subject can attack or cast spells normally. The subject of a fly spell can charge but not run, and it cannot carry aloft more weight than its maximum load (see page 142), plus any armor it wears.'
Fly and expeditious retreat don't stack. When you have fly cast on you, you use the spell to move, as opposed to moving your body. Just like when you drive a car, you use the car to move yourself, you don't actually move your body. That is why fly has a limitation on what type of movement it allows (cannot run). Expeditious retreat on the other hand is the equivalent of a booster shot full of adrenaline. It doesn't matter how much adrenline you've got in your body, it won't make the car your driving move any faster.