Often, I think, we get so into our habits and rituals and environments (all of which can be quite lovely, by the way) that go along with our important writing and yoga practices that we begin to get confused about the fact that the conditions under which we practice are not identical with the practice itself.
I wrote last about those times in the year when everything falls apart (for me the end of the semester). That time of year can be a weird environment in which to write and do yoga, much like this concrete balcony. And yet, practicing under less-than-ideal conditions, or doing a less-than-perfect practice is still doing the practice. And that's better than nothing at all. In fact, that's what the heart of practice's definition--sustaining an activity regularly so that it becomes more and more sustainable. Even when everything else gets a little ugly.
In that way the yoga mat is a nice metaphor. It's that little 2x6 space yogis carve out for themselves, which can be plopped down in any environment. Similarly, we might think about how writers can also plop down their tools wherever they are, carving out metaphorical space, even if it's just fifteen minutes of messy scribbling on a notepad.