How Fit Can You Get from Just Walking?

43 pounds in 16 weeks is 2.7 pounds per week. A pound is 3,500 calories, so 2.7 is 9,450 calories per week, or a deficit of 1,350 calories per day.

Even if he was walking a very low 3,000 steps per day previously, an extra 7,000 steps/day - about 3-4 miles - is walking for perhaps 60-80 minutes. For someone who weighs about 260 pounds, 1 hour of moderate walking would burn about 450 calories. So, assuming he didn’t do other exercise that the article omitted, the remaining ~900 calorie/day deficit came from eating less than his metabolic rate (BMR).

Walking sped up the process, but he would have lost the weight just by eating fewer calories - and food changes account for about 2/3rds of the loss.

(I’m assuming he neither added nor lost much muscle mass, which seems likely from the description.)