OK, I gotta share my experience, having read all this, and what gear is available now. I went to Philmont in 1970. Parents were divorced, we had very little money, so not much to spend on gear.
* My boots were a cheap pair of lace up work boots purchased at my local department store. About halfway thru the trek, the sole of the right boot started separating. By the end, it was flapping around so badly, I had to raise my leg significantly to keep it from catching on the ground, then slap it down - every step.
* My hat was a souvenir from the Houston Zoo - a straw fedora looking thing, with no lining. I had to cut the back off, because my backpack would rub against it.
* My sleeping bag was a Korean war era relic that my dad had stolen from the Army. It consisted of a wool blanket sewn in the shape of a mummy bag with a zipper. Over that was a thin cotton liner. We were told it would be cool at night, so I took a set of long underwear to sleep in. It didn't work - I froze my ass off every night.
* If they had invented sleeping pads in 1970, we didn't know about it. All we had was a plastic ground cloth.
* My backpack was a BSA-issued rig made of canvas, that I borrowed from my cousin. It had an aluminum frame. Probably the only decent thing I took on that trek.
* We were told to bring ponchos for rain gear, so they would cover our packs when it rained. Worked fairly well.
* There were no training hikes, etc. My first time to hump that ruck was our first day on the trail. And being from the Gulf Coast, where the closest thing to a hill I'd seen was an overpass in Houston, it was somewhat of a shock.
* Our leader had a set regimen - up at 5:30, cook breakfast, pack up and get on the trail. One morning we awoke to very cold rain and hail. Probably dime-sized, all over the ground. He was outside yelling for us to get up, and no one would get out of bed. Finally, he yelled, "I'm leaving, see ya'll at the next camp". And he left. Once we all finally got up, "just us boys" went to where we thought the next camp was, and he wasn't there. It was an un-manned camp, so we sent two guys ahead to a manned camp to get help. They found him on the mountain above us. He was up top, we were down below. He'd probably go to jail for doing that these days.
And with all that, I'll just say that Philmont was one of the greatest adventures I've ever had. I guess I can be thankful that, at the time, I was too young and too ignorant to understand how rough it was, and how ill-prepared I was. But we got our 50 Miler, climbed Baldy, and climbed the Tooth of Time the last morning. And we all still talk about it to this day.
All the advice above is solid, IMO. The one thing I'd say to really consider is the comment about using trail runners vs heavy hiking boots. Go light, and go quality.