Months not days.
Walking is a sport. Like any sport, you need to train your sport-specific muscles over months before you start getting really serious. If not, instead of getting progressively stronger, you'll just injure yourself on the first day and you won't recover. Unlike most other sports though, if you injure yourself hiking then you're most likely somewhere with no access roads and no phone signal, which means being over-ambitious is literally risking your life.
The biggest concern is ankles. Boots are heavy, and they require a different gait from shoes. You need to start with maybe 5 miles in boots, and build it up slowly.
I've currently got personal experience of this. I'm in a walking group where we walk 8-15 miles every other weekend, but we usually do "civilised" areas so we mostly wear trail trainers. I've been doing serious walking since I was a kid, so I thought I knew the score. But last year we did a few days walking where I needed boots, and I found that my ankles had weakened enough that the extra work from walking in boots has given me chronic tendonitis in both ankles. The first day of the holiday I was sore and thought I just needed to work through it, the second day I was in real pain, and the third day I could barely hobble. The physio reckons I'll be back to normal after a few months, but it's a long slow recovery process
Legs generally and quads specifically will also need some work. Going up and down hills need different muscles from on the flat, so make sure your training walks have hills. Downhill can give you knee problems too, so once again work up to it slowly.
Then there are your shoulders and back. Carrying a pack for a day is physically hard on your body, so you need to let them get used to it.
That's your physical training. Hiking also needs you to learn skills too. How good are you at navigating with a map and compass? How good are you at figuring out where you are on the map, when you're actually lost? Can you recognise poison ivy, or whatever local harmful plants there are? Can you tell by eye which areas of marshy ground are likely to be solid, and which are just water? How fast can you pitch your tent? How good is your tent pitching in gale force winds and rain? Can you pack your rucksack for best balance? All this is easy enough to learn, but it takes time to get those skills up to scratch.
And then there's your gear. You need some miles on your boots before you start, so they're worn in (and so you find if they really do fit). You need to check your tent in bad weather, because if it's prone to collapse in strong winds then you either need extra guy ropes or a new tent. If it's an older tent, it may need reproofing. So might your waterproof jacket and overtrousers. Can you get a good night's sleep on one mat, or do you need two? Is your sleeping bag right for the conditions?
The more I list out, the more I hope you see that it's not just a case of "start on Monday, be awesome by Friday". The people you're hearing about are keen hikers who are already good with all this, who are just starting a long walk. Even if you're experienced, starting carrying a pack is hard work at first, which is why you start more slowly. As someone who's completely new to walking though, you must not attempt this. You will be endangering your life, the lives of the people you're with who will need to help you, and the lives of the emergency services personnel who will need to retrieve you from wherever you get injured.
Please do get yourself walking. It's amazing fun. And when you know enough to not be too dangerous, by all means get yourself out on trails with a pack. Start with days, then weekends, and stretch it gradually. And when you're ready, get yourself onto that epic trail you've been working up to. But not today, because you aren't ready.
Edit: The OP apparently does not need "new walker" advice, because he already has some experience. Maybe he's OK, but I've seen a number of people get into trouble in the hills because they didn't know enough to be safe. I'm keeping my post in place, because it is still an appropriate answer to the question.