These were based primarily in Himachal Pradesh, where the treks were as gruelling as the terrain.

Gruelling means extremely tiring and demanding. Rigorous means extremely thorough, exhaustive, or demanding, making it the closest synonym. Egregious means outstandingly bad or shocking; unctuous means excessively flattering or oily in manner; ominous means giving the impression that something bad is going to happen. Therefore, rigorous best captures the meaning of physically demanding and exhausting.