I think it is good to either increase distance or speed or incline when you are trying to improve yourself when walking.
If I am walking on the treadmill, I pick up the pace. I also will put the incline to 5% or more and walk that way at the same pace.

Another thing that works for increasing the amount of calories you burn is to do intervals (HIIT). Walk fast and then slow down after a few minutes. It'll tire you quickly, but I have found it increases my VO2 max quickly and I get in shape faster.

Btway, I just want to remind Cy and a few others that when you are making snide/uncalled for remarks on what I say, then I will not reply to you. Cy knows he is on my ignore list, so stop trying to troll me.