ExpressionIntended conditionGrammatical attribution
sālat ... al-abāṭiḥRapid continuous travelFlowing assigned to valley grounds
wa-shtaʿala al-raʾsu shaybāSpread of gray hairBlazing assigned to the whole head
Both constructions enlarge a particular condition by attributing an evocative action to a wider scene or place. The result is an impression of pervasiveness and intensity.
The comparison is one of rhetorical method, not literal subject matter: travel and terrain are distinct from aging and gray hair.