Yes, it is a good idea to hike from jiankou to Mutianyu. You can fo it. You are right. It is about 10km. Possible for hiking. In fact, it is a very popular hiking routes favored by backpackers. You should take Bus 916 at Dongzhumen Long-distance Bus Station to Huairou first. Then change the bus to Jiankou (西栅子村口, this is the village name). Depart early in the morning and get to Jiankou to start the hiking at 10am. You may arriving in Mutianyu at 5pm or 6pm. If you will get back to downtown Beijing, you should hire a van or share with others. 916 bus is not available at 7pm. Time here is just for reference. It is up to your hiking speed and other arrangement. Enjoy the scene.