Possible to hiking between Jiankou and Mutianyu in one day, but you need to get up early for public transportation that morning. For example, if you arrive in Dongzhimen at 7am, take bus 916 to Huairou (get off at 青春路北口) and then rent a taxi to Jiankou first. Generally, you can reach jiankou before 10:30am. Then start hiking from 11am or so. Remember to asked the taxi driver in advance to wait for in Mutianyu end at about 5:30pm, depending on your speed. I think you may finish the hiking at that time. Then take the taxi back to Huairou, where you get off the bus 916. Then wait for the bus back to the city center. Talk about the price with taxi driver in advance for trip back to Huairou from Mutianyu.

Well, I suggest to Jiankou first because Mutianyu is closer to Huairou. If the taxi driver would like to wait in Jiankou end, that's fine too. The last bus back from Huairou to the city is at 19:50. Better to get there earlier.