At 24 mpg you are probably averaging about 35 mph, so to burn a gallon with the engine under load, doing useful work, takes about 45 minutes. At idle, hot, the engine might have used a quart and a bit. What the enrichment did in the first few minutes was burn a bit more.
The catalytic convertor doesn't like to be idled. It fails to reach its optimum temperature, and that coats it internally. To clean it requires full throttle - the Bosch system like many others goes intentionally rich at WOT for power, cooling, and to provide enough fuel to heat the convertor past the normal self-cleaning temperature. Doing that negates a low of fuel saving techniques.
As another poster said, use a block heater, and don't idle for more than a couple of minutes.