Well, not that I know for sure of course but here are my guesses:
Walking inside our ship? - Very soon, 2015, maybe as early as spring (or summer) as this is by far the easiest to make out of all the places you can walk around.
Walking around on parts of the stations? - Less soon, I still think 2015, maybe summer or autumn. A lot more complicated given all the scenery that needs to be made and implemented properly.
Walking on asteroids and moons with no atmosphere? - Unlikely to be soon, I think late 2015 maybe or early 2016. Obviously more complicated than a station, but from what I read these will come first as a way of testing the planetary generation and gameplay mechanics without the need of a complicated atmosphere or lighting or flight physics.
Walking on actual planets? - Nowhere near soon, 2016 probably. Obivously a very complicated feature to get right on such a huge scale.
And everything else such as water (and underwater) environments ... far far in the future, 2016, 2017 who knows.
Just my opinion.