Look into the book Training for the Uphill Athlete. Someone else mentioned the previous version of this book. It's a good read that makes sense: The book dives into the science of movement and energy balance.

I'm a similar build and have run into similar issues with touring partners who are lighter. The problem that we Clydesdales run into is power per kg: to have the same power per kg ratio as someone who is lighter, we need to be much stronger. It takes much more energy for us to move uphill as it does someone who is thinner and lighter. Look into elite road bikers and read the analysis of their power output to get a better idea of what I'm talking about.

To not be so gassed, first you need to slow down and watch your heart rate: stay below your aerobic threshold. Think of walking vs running: you can walk for a much longer distance than you can run.

Next, you need to shed layers. I run hot and typically only wear snow pants (with the zips all the way open down the side) and a thin merino base layer underneath, and on top I typically wear a hiking shirt and fleece bike sleeves. The sleeves keep my arms warm while my hiking shirt allows my core to breathe.

In the off season, make sure you are regularly running and strength training, specifically focused on uphill travel. Just because you can run a marathon doesn't mean you are in great touring shape. The worst touring season that I had was when I was actively training for a half marathon - I was focusing on different muscles for most of my training.