The mistake almost everyone makes
Treating the loop as a list of four or five stops to clear. It is roughly 230 to 250 km round trip around three places: Þingvellir, Geysir and Gullfoss. The driving alone is about three hours. Add a fourth stop and a meal and the day is eight, often finishing in the dark in winter.
The honest choice before you leave: three stops done properly, or four done at a jog. Everything on this page, parking, food, daylight, depends on that single decision.