[kragen]

Most geothermal energy comes from the decay of radioactive elements in the Earth's crust, although heat from Earth's formation is a non-negligible fraction of it. If you check out the web site of Iceland's geothermal energy agency, I believe they do have a calculation there of the sustainable power level that could be extracted (without cooling down the crust), but I don't remember if they're currently above or below it.

If I recall correctly, however, the fossil heat trapped in the crust under Iceland is several billion years of the sustainable extraction rate.

And, on the third hand, even if you only extract energy at the rate that radioactive decay produces it, in only a few tens of billions of years, most of the radioisotopes will have decayed away if you don't replenish them.

You are correct that the crust is heated more from below than by the Sun. That's why the bottom of the crust, where it contacts the mantle, is hotter than the surface.