Lenishta,
Glad to hear you are still keen to do the multiday stuff. Most people expect packs to fit perfectly and not leave them feeling sore and stiff after buying new, hiring or borrowing etc, but IMO it's as much about conditioning your body to carry 15-20kg's on a regular basis.
Getting the correct fit is an obvious start point but like learning to ski, it takes a season or two for all the different muscles to strengthen and work together before you think less about the discomfort and more about the enjoyment. Think about how often you put that amount of weight on your shoulders on a regular basis and walk all day.
I don't believe the majority of people start out with the perfect fit and never get some kind of fatigue or soreness. Perfect world scenario and great if you have achieved this.
I have had 6 or 7 packs over the years (over 30 years) and all of them gave me some kind of soreness in the breakin period. The more I used a particular pack the better it felt which is simply due to breaking the harness down a little until it fit my body shape.
My favourite packs seemed to hurt the most initially but perseverance paid off. Mostly sore shoulders to start with but moving the load around (on and off the hips etc.) during the day helped. Thinking you're going to buy a pack off the shelf, load it up, and have a trouble free introduction is a little optimistic if you haven't carried heavy loads in the past. If I were to buy a new pack now, my break in period would be very minimal as I'd say my body is conditioned already.
Keep at it and I'd be interested to hear more of any future trips and the outcome.