The beach wearing a loaded backpack. Hiking nearby low soft sandy dunes in FL when I lived in Tampa, Pt Charlotte/Punta Gorda, and Ft Myers and stadium/building steps was great backpacking training.
Backpacking training isn't all about having to hike on a trail or training on great ascents/descents on trails. IMO, it's an accepted fallacy that the best training for a LD hike is hiking. This does not have to be the case.