There are lots of references to situations experienced by the men being described in this poem that are not as romantic and wonderful as the poet wants the reader to believe.
Living in "the land where the mirage lies" refers to surviving in extremely hot and dry conditions, where heat waves radiating off the ground surface give the false appearance of water, that rarest of elements in the desert. There is nothing glorious about living under those types of circumstances!
In the same sense, fighting drought, fire, and flood "in the fields farthest out" describes dangerous and difficult work far from any help or equipment to make the battle easier.
On the other hand, each man appears as "emperor" when he "ride(s) with a gallant bearing where every saddle's a throne." To say that these men "rule the world when they ride" is certainly a glorification of their continuing battle against the elements present in "the open spaces."