I spent a good 12 years as a "professional rucker" and I agree with almost everything Al Ciampa has said in his article; I would emphasise and add a couple of things though:
1. Don't carry useless weight unless you already have everything you might need. There have been several high profile cases in the UK of soldiers, including Special Forces, dying of exposure/ heat exhaustion whilst out training alone with a rucksack full of sand when they were injured and succumbed before help arrived. I saw plenty of less dramatic examples of the same phenomenon with my young soldiers. Even if you do not go to remote areas, think about how long it might take a passerby to find you if you were to break an ankle and land on your cellphone. As a minimum I always required my guys to carry extra water, warm clothing, waterproof suit, hat, gloves, and a bivvy bag/survival bag/trash bag. Some extra food is always useful, as is the means to make a hot drink if you are in a cold climate. (or some shade in a hot climate-mylar blankets work great)
I am not generally a fan of carrying extras when running etc, but if you are carrying 15 kg you might as well make 5 of them useful kg.
2. What we called "admin" (personal administration) is 50% of the battle. Even if you are super fit, if your feet are blistered, your crotch is bleeding from chafing, and your pack fits poorly and causes pain, you won't perform, so pay attention to these things. That said, be aware that you will chafe/blister/rub/ache to begin with as your body toughens up, just minimise the needless suffering through careful prep.
On packs, it pays to invest a little. Be wary as many civilian orientated packs, especially cheaper ones, are not designed to carry these kinda of weights and will hurt/fail. Packs aimed at the military and serious outdoor folk (especially climbers/alpinists) tend to do better. Hip/waist belts, when used correctly are a massive aid and will save you back pain; you might rarely see soldiers using them but this is only because they are impossible to fit over webbing and body armour.
3. Be humble and start small. Whilst I have found Kettlebell training (in particular swings and TGU) to be the best non-specific training for rucking, you will still need to develop it as a skill and toughen your body as per point 2. Given the additional load on your body for a long time, it is relatively easy to give yourself injuries overdoing it. Take it easy.