Using my Ecobee Suite, set the default hold time to 2 Hours (in Ecobee (Connect) or in the Device Handler). Then click the fan button to “On”. This will create a 2-hour hold with the fan on.
Not sure what you want here - why don’t you just set the Away program to 78?
Or if you want to wait until the temperature hits 82 before cooling to 78, you can set the Cooling Delta to 4+ degrees (you have to set this on the thermostat preferences), and the cooling temperature to 78. This will wait until the temp hits 82, then cool to 78, and turn off again, until the temp hits 82 again.