Really rough, but heres my attempt. 
Assembly: a central bar of alloyed aluminum, hollow and reinforced internally, coated in a lightweight foam and rubberized polymer, for enviromental protection and buoyancy. The hoof segment has a simple shock and soft cleated soul for traction. 
The the upper leg is attached to a swing bar, pulling the limb up and folding when the proper motion is made, also allowing it to extend naturally. The inner workings are tied to the swing bar to create a smooth, predictable motion, multiple slides keeping the limb firm but easily moved by the wearer. 
Runic enchantments increase the tensile and shear strength of the frame, and a set of four runes act as a lock when in proper proximity, holding forlimbs in their folded position and lessening the strain on the wearer while doing so. 
To release the lock, one simply has to extend the limb and seperate the rune on the harness from the three in each leg, causing them to release.