If you have a lot of energy downstairs, and are not afraid of thin air, you might want to consider Everest Marathon. The start off is at Everest Basecamp at 5362 meters, and it ends in Namche at 3540 meters. It ascents 2777 meters, and descents 4579 meters. If this is not enought for you, they also sport the 62 K Extreme version, with a detour up to Nha La and Machermo at about 4450 meters.
Read more at Everest Marathons website.