So I took the dogs for a hike today and wore my pack fully loaded.
I was pretty impressed. I did well. Of course, once I add about 30 degrees and much thinner air, I'm sure it will be significantly more difficult but I really think I'll be fine.
With the pack on and the higher altitude, I imagine that it will feel like hiking when I was out of shape. The pack weighs exactly the same amount as the weight I've lost over the last 6 months.