@halfeye:
You're right that the interior of our planet is enormously hot -- moreso than the surface of Venus, if you go down far enough.

Our lithosphere is about 2% of planetary mass and has an average temperature of around 400° C. This still is in the same order of magnitude as my earlier very rough estimate (1% at 300° C). That is, around a hundred thousand years to cool down.

If you want to bring this value down, I'd suggest going after the heat radiation and heat storage (as opposed to temperature, which we've been covering so far).

I'd be surprised if we end up at 100,000 years on the dot; still, you need to somehow decrease this by four orders of magnitude to make the endeavor worthwhile.