Did this loop with my small dog as an overnight trip, starting with the AT (south) to Mount Prospect, then staying at the Sperry Road campground. Completed the second half including Mount Greylock the next day. Unfortunately there were no views from Greylock due to the very foggy, misty weather, although it did make for a pretty quiet hike. Was definitely made more difficult by having an overnight pack, but I can see this being doable in one day without the pack as long as you begin early and are in really good shape. The trail was well marked the entire way, especially the AT sections.