Learn Spanish (Latin America): Estábamos resistiendo el aire escaso.

Both translate to "we were enduring," but using the imperfect of estar + the -iendo form (the past progressive) puts extra focus on the action actively being in progress at that exact moment in the past. Resistíamos is a bit more general, like saying "we used to endure" or "we were enduring" over a broader timeframe.

I thought resistir meant "to resist." Why is it translated as "enduring" here?

While it does mean "to resist," resistir is also heavily used in Spanish to mean "to endure," "to bear," or "to withstand." When talking about harsh environmental conditions, physical strain, or difficult situations (like a lack of oxygen), resistir is the most natural choice.

Could I put the adjective first and say el escaso aire?

You usually place the adjective after the noun (el aire escaso) to give objective, descriptive information—in this case, stating the factual condition of the air. Putting it before the noun (el escaso aire) would make it sound more poetic or literary, emphasizing the scarcity itself rather than simply describing the air.

Since this likely describes high altitude, could I say el aire delgado for "thin air"?