Expeditious Retreat says
This spell increases your base land speed by 30 feet. (This adjustment is treated as an enhancement bonus.) There is no effect on other modes of movement, such as burrow, climb, fly, or swim.
The fell flight invocation states that the fly speed is your land speed.
FELL FLIGHT Lesser; 3rd When you use this invocation, the powers of darkness bear you aloft as you sprout a streaming, winglike cape of shadows. You can fl y at a speed equal to your land speed with good maneuverability for 24 hours.
Does Expeditious Retreat affect the warlocks fell flight invocation? Would the order matter, for example, should the land speed be increased first and then fell flight be invoked?