The distance.
The first leg, leaving Qiemo, cycling the first part in the outskirts of the Taklamakan and to reach the actual high plateau is a distance of roughly 300km.

Ahead of us is now a stretch of 450-600 km of roadless wilderness. The distance of this leg of the journey is uncertain and has to be regarded with care. Unpredicted obstacles may interfere with our planned way towards the south: a wild river which is difficult to cross, big swamps, areas with deep sand, steep and difficult passes may force us to make long detours. When we finally reach the first settlement- Shuanghu the last leg begins and there are approximately 900 km left to Lhasa.

Time wise this is roughly: one week from Qiemo to the plateau, five-six weeks through the roadless wilderness to Shuanghu and another two weeks reaching Lhasa.