If that's what happens when you carry 19 lbs, you do need to work on physical conditioning. Carrying the extra weight is one way, running, power walking and lifting weights is another -- work on overall physical conditioning, not bulking up. But don't do the same thing all the time.
You have one set-up now, 19lbs and it makes you collapse -- work with that because it seems that your primary goal is to carry that pack and feel good doing it. But not all the time. . .
For another set-up determine what you require for your day hike and take just that (possibly just shy of 10lbs, with the idea of working on speed. Don't forget to drink the water during the hike to further reduce the pack weight as you move. Take some energy bars (100 calories/mile, no more).
Hit the gym or otherwise get a workout which focus's on leg and back strength -- once a week.
Look at nutrition too.

I'm in my mid-50's and work out regularly, over the last 6 years my leg strength and endurance have improved immensely. I also work on upper body strength including weight training, and good old push-ups and pull-ups. Physical conditioning should be part of the lifestyle of anyone concerned with being prepared/equipped to survive.

$.02, this is your life. Now get down and give me 50 (40?. . . okay, let's start with 20)