If you walk 20,000 steps daily and consume less calories than you burn daily, you definitely will lose weight. But if you walk 20,000 steps each day, but consume a caloric surplus (or more calories than you burn each day) than you will gain weight.
Getting 20,000 steps per day is exhausting, but it's the good kind of exhausting. The kind of exhaustion that brings with it a sense of satisfaction. Every night during this week-long challenge, I crawled into bed earlier than usual with sore and tired muscles—and I loved it. I had pushed my body and it had kept up.