Stronger is relative. Their hind-legs tend to be stronger overall. Forelegs is about normal. Are about normal.
There’s the whole nature thing and connection to the land, but that’s mostly just cultural.
For the most part earth ponies have endurance on their side. An earth pony at the peak of physical performance can run for over twelve hours straight. I think it was twelve… Unicorns and Pegasi can’t even approach that.
In fact, no land-based creature in Equestria can.