Adding hills or challenging terrain will speed up the rate at which you use up calories.

That climate and challenging terrain make it an ideal training base for athletes.

The challenging terrain demands a certain toughness.

It's a remote region of rich landscapes punctuated by cypresses, olive trees, and mossy chestnut and beech forests filled with wild violets and orchids; gorgeous albeit challenging terrain.

It will survey the environment as it descends, adjusting its course if necessary, allowing it to operate in more challenging terrain.