It appears to be set in the mountains to the East of Anchorage and then in the Rabbit Creek/ O'Malley Road area. While the flat-land portions are possible, there is no way these mountains could be the same ones that are there.
I understand that a lot can change in 55 years (and that the Fallout 3's alternate reality timeline diverged from our own some time prior to the present). I also realize that it is supposed to be a simulation and the General himself was dissatisfied with the simulation's accuracy. But really.