My guess is, it was the return journey up from the Humboldt River to the saddle below Mawson that caught you?
Always a good idea to check the contour lines on the map, and weigh that up against your hill climbing experience/fitness level for such a walk.
I know plenty that could do it in 4, the track along that ridge is a great place to spend some time though
Did you get up to Lake Belton as well?
sbs