An 8km walk at a brisk walking pace is a little under an hour and a half (average 10k running times is about an hour). I get out of work at 5pm, and it's officially dark by 5:30pm. I agree with a lot of what you're saying, but this one is literally impossible for anyone in the northern latitudes (New York, Chicago, Seattle, London, Berlin, Hong Kong, even San Fransisco is officially dark before 6pm this time of year).
Instead, I'd suggest focusing on taking vitamin D supplements if you don't have a lot of sun then taking a vigorous walk whenever you can. You don't need one more thing to be anxious about!