Synthesis of hydrogel networks capable of accurately replicating the biomechanical demands of musculoskeletal soft tissues continues to present a formidable materials science challenge. Current systems are hampered by combinations of limited moduli at biomechanically relevant strains, inefficiencies driven by undesirable hysteresis and permanent fatigue, and recovery dynamics too slow to accommodate rapid cycling prominent in most biomechanical loading profiles. Here, we report on a novel paradigm in hydrogel design based on prefabrication of an efficient nanoscale network architecture using the melt-state self-assembly of amphiphilic block copolymers. Rigorous characterization and mechanical testing reveal that swelling of these preformed networks produces hydrogels with physiologically relevant moduli and water compositions, negligible hysteresis, subsecond elastic recovery rates, and unprecedented resistance to fatigue over hundreds of thousands of compression cycles. Furthermore, by relying only on simple thermoplastic processing to form these nanostructured networks, the synthetic complexities common to most solution-based hydrogel fabrication strategies are completely avoided.