Lightweight, you don't feel anything from rocks, there is a plate in the midsole. It is not waterproof, not for many kilometers. For a 20k, I think it's borderline. After that, you want it to have "meat" for cushioning. In general, I wear them when I have interval uphill runs on Hymettus on rocks. Also, obviously a personal estimation, they've been very convenient for me on downhills, the foot doesn't move inside and because they're lightweight, they're even faster. Keep in mind that they're not a tank like a well-known brand around the perimeter. Therefore, you feel small stones on the side but not to an annoying degree.