National Geographic Traveler included Beijing Hikers in their 2017 China edition, mentioning "for fun and energetic treks in the countryside, contact Beijing Hikers".
Thanks, National Geographic Traveler!
Scans from the guide are included below.
Experience: Hike Into the Hills With Beijing Hikers
The city of Beijing is remorselessly flat, but the environs of the municipality are hilly and even mountainous, with hiking trails winding around gullies, ravines, lakes, and remnants of the Great Wall that beckon outdoor lovers.
For fun and energetic treks in the countryside, contact Beijing Hikers (tel 010 6432-2786, beijinghikers.com, $$$$), which offers regular weekend, and some midweek, hikes, as well as some overnight trips. Reservations are a must, and prices include transportation, guides, and snacks. Hikers meet at the Starbucks in the Metropark Lido Hotel (6 Jiangtai Lu, Chaoyang) in the far east of town.
Beijing Hikers can arrange tours for small groups, as long as you give five days' notice, and also conducts tours to other parts of China. Kids are welcome.