I’m from a town that offers a lot of high country backpacking trips, often a lot of teenage girls from back east who have never seen a mountain. A month long trip used to start out with 70 lb packs. Its nice to stroll around the park with light weight, but if you’re physically able there is no substitute for a heavy pack - just like the little girls from back east do.
Having said that, I used to hike 14ers in Colorado on weekends and just one quick paced 6 hr trip with a light day pack got me in better cardiovascular shape than slow jogging 5 miles every day, and built up mountain muscles almost as well as frequent backpacking, but weight bearing shoulder and hip muscles need heavy weights.