400 calories is not enough for me for any meal, especially breakfast. I would get hungry before lunchtime.

Whatever your total caloric goal is, it's best to have more of the calories at breakfast and lunch and less at dinner, so you don't wind up so hungry late in the day that you overeat.

I generally walk at a pace of 3 mph when I walk for exercise. On the treadmill, I do intervals by increasing and decreasing the incline. Outside, I walk routes that include hills.