It is very hot and humid (we clocked 28degrees and 94% humidity) and this means constant sweating and the need to drink lots. Also clothes don't dry very easily.
The Pinnacles climb is only 2.4km distance but is a height gain of around 1100m over very steep and difficult ground. The last 200m climb is ladders and ropes. It took 4 hours and the group that made it were pretty fit. The return trip took me over 3 hours and some of the group took 4 hours longer and arrived back in the dark. This is an extremely tough day particularly in the conditions.
Mount Kinabalu is also a tough 2 days and the journey to/from base camp was in torrential rain. Thankfully mornings are usually sunny and the summit climb was magnificent.
Food is basic and pretty much the same every day so don't go fot the food.
Pack light - everything gets carried a lot during transfers (airports, buses, boats)