It takes five hours of solid travelling through intense heat (with one saving throw per hour -- these are saves 9, 10, 11, 12, and 13 so the DCs are 22, 23, 24, 25 and 26; Planus still provides everyone with a +2) to reach the distant volcano. If you can climb the steep slope, you should be able to get out of the oppressive heat. You are all now exhausted.
Without equipment, the climb check is DC 14 for each individual. (We'll treat it as one check to represent all of the hardest spots heading up the mountain.) Each failed climb check will represent unnecessary work and time in the heat, calling for an extra save versus the environmental hazard.