This is an early milestone in the "On-foot Exploration" or "walking about" statistic and is obtained once you have travelled 40ks. You can check your progress by pressing
and looking at the Milestones screen. It is the top entry in the "Journey Milestones" section.
I'm not going to go into details of how to walk around (try using
) but if you're in a hurry to complete this then find a nice planet (not too hot, not too cold and not radiated) and walk until your heart is content.
Note. The achievement tracker (percentage complete on the Xbox) does not work for any of these achievements. The achievement is for reaching the target milestone.