Start with 10 - 25 pounds, depending on your current fitness activity level and bodyweight. When in doubt, aim for lighter weight and longer distance on your first few rucks. If you can't maintain good posture for the entire ruck, lower the weight and build up over time. Work your way up in increments of 5 pounds each week, or as you see fit. Listen to your body, stretch, hydrate, and adjust as necessary.