Just like you, for whatever reason, I have made 15 my slowest walking speed. If I slow lower than that, then I make myself pick it up. I use that rate for when I just need to get some walking done but am just not up to pushing it. When I'm really working, I try for about 13.5. I'm working at bumping that up with some intervals as fast as 12. SideStitchWalker, the best advice I can give you is to listen to your own body and do what works for you. A 'comfortable' pace to me means pushing enough that it's a nice brisk walk, but not hard enough to wear yourself out.
A 14 min walk pace will put you finishing a full marathon in about 6 hours, Disney has a 16 min per mile time limit which puts you in at 7 hours, so 15 min is a good goal to shoot for. When I do the marathon with Erica I try to keep us on a 15 min per mile average pace even with all her stops for potty, petting animals and character pictures.
I had a patient yesterday who was a long time runner that had been forced to quit running because of knee problems. He had switched to swimming for exercise but wasn't able to do it often enough or long enough to keep his weight down like he could with running. We talked about walking as an alternative. He asked how one gets started race walking. I had been doing the class at the health club to teach basic technique but had gone to the group Thursday morning walks instead.
Just how does a person learn fast walking? To start with as with running you have to build a base of strength and endurance with walking. Daily walking is best and at least 30 minutes of motion. While walking you start incorporating race walking techniques. If you just try to start off race walking is feels awkward and very quickly your shins will begin to burn.
First off get your arm swing more to the back, like you are elbowing something behind you, not an exaggerated motion but smooth and flowing. Then walk with your feet in a straight line, walk on a line in the road or imagine you are walking one and keep your feet on the line when you bring your foot down. You have some hip motion as you swing your foot around to get it into the front. This is way more efficient than the duck walk foot plant that most people do when they walk.
Walking actually puts more muscles in motion than running does. Walking is a whole body workout using your entire leg, hips, glutes (Walkers have the best butts) lower back and abs. The arm swing tones the upper body. Running is about raising your knees in a piston action, walking is moving the leg as a lever. Because of that a walker doesn't work the quads as much as a runner does. You can do leg raises on a machine, pool sprints or just sitting in a chair and raising your legs up and down for three sets of 25 to work the quads. That will keep you from getting an imbalance. Runners on the other hand need to do more ham string and shin strengthening than walkers since running doesn't work those areas as much as the quads.
As always, any exercise is better than no exercise. Get up and get moving, winter is coming on and that is absolutely the worst time to get out of the habit of regular exercise.
Thanks for reading.
Walking Panda
I'm hoping to do the 1/2 in Dec at 15mpm and am wondering if that is reasonable. At this point, I'm just confusing myself trying to figure out what to shoot for. Help! Please! If it helps I did the TOT 13K in 1:55:10 chip time.
I walk, I jog, I racewalk (kinda); as long as I keep moving I figure I'm okay. 
Now if this wretched weather will just clear up a lttle so I can get outside and 'walk a line'. Thanks for the great descriptors. 
