Hello - I don't have an answer for you, but I do have a 99 ES300 with 139,000 on it and I get the exact same high idle at startup. It jumps to about 1600 rpm and slowly comes down to normal in just a few minutes. My combined gas mileage mostly city is usually 21-22 mpg in warm weather and 19-20 mpg in cold weather (below freezing). I believe this is normal gas mileage, or very close to it.
So I'm not sure that your high idle at startup and the bad gas mileage are related, at least my experience doesn't support it. If you find an answer for the idle please post it here, I would love to know if it's a problem and what the fix is.
Jason