I, too, am just getting into backpacking after years of day hiking. Here are some things I've found to be helpful so far:

  • Training - there's training for strength and for endurance. Both are important. For endurance, start doing long walks around your neighborhood or local parks. Unencumbered by a pack, you should be able to do 10+ miles with relative ease. Doesn't have to be non-stop or at a fast pace. I think "10" is really important because it's the first double-digit number to break. It's a mental thing. When I was first putting on long miles while day hiking, I would go for 6, 7, 8, or 9 but 10 just felt like it was too long. Double digit mileage when we first are starting out is the physical manifestation of our mental limits. We don't do it because we believe we can't. So break through that physical limit and you'll find you've broken through that mental limit. It's a huge feeling of accomplishment. Then start carrying a pack - even a decent day pack with several full bottles of water to get used to hiking under a load.
  • Gear - I assume you have your big three already? Pack, tent, sleeping bag. If not, get your bag and tent first, then get your pack. Go to your local REI or sign up for virtual outfitting and they can help you dial in the right pack. Between @hikermor  and @Philreedshikes  you've gotten some great advice (even with the conflicting theories on a multi-tool LOL). So all I'll add there is to feel free to carry a light(er) weight luxury item or two. Yes, you want to lighten your load as much as possible but you'll want something to keep the experience enjoyable. Maybe a pack of cards or a book. Me? My non-negotiable luxury items are the Nalgene ultra-light flask (for medicinal reasons only, of course), a couple cigars, my journal and a pen. I do have a new iPhone lens that I've not tried out yet because I don't want to carry my dSLR kit.
  • Fire stuff - I've been having fun playing around with... oops, I mean "scientifically testing" different DIY fire starting materials. I've settled on smearing Vaseline on those square cotton makeup pads, then rolling them up. They work well and are less messy  than cotton balls. I had to buy my own package of them after my wife started asking why I needed to use her makeup pads. She wasn't happy to learn that I was taking them just to light them on fire. HA HA. As I say, once a 12 year old, always a 12 year old.
  • Food - get your cooking kit sooner rather than later. Then start practicing with it. Even if you're only going to rehydrate packaged meals (ahhh, to be young and not worry about high blood pressure again...), you will want to get used to using the kit. Plus you should also figure out which meals you like and which ones you don't. Not all dehydrated meals are equal and with so many choices on the market now, there's no reason to bring meals that you dislike unless you don't try them first.
  • Coffee - I'll save the most important for last. Decide on your coffee-making system and, like food, practice at home where a bad cup of coffee won't ruin your entire trip. Okay, maybe I am employing a bit of hyperbole here but if I am, it's only a bit of it  🙂  I have both the Ultralite Java Drip from GSI (and really like it) but I also use the Aeropress at home and have decided that it's worth bringing it in the pack. Yes, a poor cup of coffee at the campground becomes a great cup of coffee just because of your surroundings BUT a great cup of coffee at the campground becomes spiritually transformative.

Lastly, the one thing I've found is that, although following the YouTubers has given me a lot of great advice, it's really easy to get sucked into the idea that, if you're not spending $600 on a 1.5 pound tent, $500 on a frameless ultralight pack, $600 on a down quilt that packs up to 4 cubic inches, and saving 6 pounds of weight by not carrying water but rather licking the dew off of leaves in the morning, then you're doing it all wrong. 

It will take time (I am finding myself) to totally dial in the gear you want to carry. Get what works for you, don't feel bad returning an item to get something different. 

“Between every two pine trees there is a door leading to a new way of life.” (John Muir)

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