Pokemon GO 50km rewards changed in August as Niantic Labs slightly altered the rewards for players that walked 50km in one weeks time using Adventure Sync.
Adventure Sync allows you to count steps and distance even when Pokemon GO isn't open or being used. Here are all the rewards earned for 50km.
Pokemon GO 50km Rewards: All-New Rewards
You first get 20 Poke Balls for 5km. Then you get the 5km reward, 10 Great Ball, 1000 Stardust, 3 Rare Candy or 3 Silver Pinap or 1 5km Egg when you hit 25km. Finally, you receive 5km rewards, 25km rewards, 5 Ultra Ball, 5000 Stardust, 5 Rare Candy or 5 Silver Pinap or 1 5km/10km Egg when you walk 50km.
Sadly, the rewards aren't too impressive for someone walking 50km in a single week. Those players already play the game a lot and don't need the usual items. The most needed items are Sinnoh Stones which are far too uncommon. That would be a more rewarding item to receive.
Photo courtesy of Niantic Labs