I'm trying to use AirPort Untility to configure the old APExtreme to extend the network, but it won't let me.
"Extend" is only used if the two AirPort Extremes are connecting using wireless only.
"Create" is the correct setting to use since the AirPort Exremes are connecting using Ethernet.
Configure the second AirPort Extreme to create a wireless network using the exact same wireless network name, same security setttings and same wireless password as the "main" AirPort Extreme and make sure that you are choosing the Bridge Mode option for the second AirPort Extreme before clicking Update to save the settings.
When you do this, everything will act like one big wireless network and computers will automatically pick up a signal from the AirPort with the strongest signal based on their location.
This Apple document more details on this....which is called a Roaming Network.