I had the luxury of having a brother who walked the Kokoda Track with Kokoda Trekking (KTL) last year, so he was able to give us some really sound advice in advance.
The first piece of advice is to get fit.
You will spend one 8 hour day walking solidly uphill with your (about 8 Kg) day pack on your back. Other days you will spend a very long time walking downhill which is actually harder. Do lots of walking up and down steep hills.
The second piece of advice is to get your malaria medication
This isnt difficult just go to your doctor.
Most malaria medications need to be started a couple of days before you head to New Guinea check with your doctor.
Malaria is one of those diseases that mutates regularly, so get up to date info and get the right pills. Different doctors will prescribe different medication so dont be surprised if your trekking mates have pills different to yours.
The malaria medication will be a PROPHYLAXIS. This type of medication will not cure you if you get malaria. Also, it will not guarantee that you wont get malaria. All it will do is dramatically reducing your likelihood of getting it.
Probably all of us have heard stories about how dangerous Port Moresby is. I never witnessed any trouble, but the hotels are surrounded by high walls topped with razor wire, and if you try to leave the hotel grounds, the hotel security will physically stop you if necessary.
However, once you leave Port Moresby and start the trek, the question of security seems to vanish. The locals you encounter are some of the nicest people you could ever wish to meet.