Mall walking and treadmills are what walkers use to stay warm and dry in the winter. Some malls have specific walk hours and some are open early just for walking. Most mall walkers are seniors and many have infirmities making the mall an inappropriate place to do speed work. A number of Anchorage Team in Training walkers did 14 miles in the mall preparing for the Honolulu marathon. Not my favorite, but a good choice in sub 20 degree weather.
Treadmills are a good substitute for outdoor walking especially in the dark and wet.
You do, however, need to spend some time on the road if you are preparing for a road event. Treadmills can be used to add strength to your workouts by increasing the incline. And also speed to your workouts by increasing the pace appropriately. But be warned, I have seen a number of injuries from walking on an incline all the time so it is best to vary your routine. Volkswalking is a non competitive, non-timed walk.