What should I do in the first 24 hours after a hard 18 holes?

The first 24 hours decide whether you wake up loose or wrecked. Walk a little after the round instead of sitting down cold, rehydrate properly, get a real meal with protein, and spend 10 minutes on hip flexor and thoracic mobility before bed. If anything is sharp or one-sided, don't stretch into it — get it looked at.