when I was racewalking in the 1980's, I learned that losing 10 lbs equated to approx 1 minute per mile. so your speed will "naturally" increase as your weight decreases. other than that, you eventually might want to alternate days of "long slow distances" with interval training, where you walk faster (fastest) for 100 steps, then slower for 100. these are great for weight loss AND increasing your "slow" speed.