When walking around my city, I've noticed debris and garbage along the roadsides and in the ditches. It sucks to see the environment in such shape. The sidewalks are also pretty narrow in some parts, and in other places they just end on one side, so you have to cross the street to continue walking on the sidewalk. Also, there are some areas that are just really plain and lack greenery. I think that if there were either larger areas of grass to walk on or larger sidewalks (and sidewalk present on both sides of the road) it would make the walks more possible. Also adding more green space would make the walks more enjoyable. Either way, my favorite places to walk are nature trails, and it doesn't get more green than that, but just adding a little more of the green beauty to our constructed environment would make it all the more appealing and inviting, and would encourage people to go out and take walks.
I think that I'm lucky because where I live, there is more green than, say, a city of skyscrapers and such. Parts of my city are still not as developed as they could be, so the green and trees are still there. But it is a constant struggle to keep it that way.
I hope that others will see how important it is to have nature around when we walk around our cities.