@mhsc:
When you say it took you several seasons to get really good at carrying a heavy pack uphill, what does this mean exactly?
A strong base is key, but that should involve a mix of both long slow distance runs and strength (via a weighted pack), correct?
If you’re talking about doing aerobic hikes with a weighted pack, please have a read of this topic where I responded to a similar question, and this post in the Military section of the website, which does a much better job of explaining the same concept than my post.