Preheat the oven to 350°F. Pierce each potato several times with a fork, then wrap them individually in aluminum foil and bake until easily pierced with a fork, 90 minutes or longer, depending on size. Remove the foil, let cool slightly, then halve each potato lengthwise and scoop out about half the flesh with a spoon, taking care not to break the skin. Turn up the oven to 425°F. Lightly grease the potato halves with the olive oil, season generously with salt and pepper, place on a baking sheet, and bake for 6 minutes more. Reserve.