How long would it take to walk 850 meters?

That depends on how fast you are walking. I could give you the formulae for working this out if it helps.

Time taken = distance / speed

Speed = distance / time

Distance = speed x time

Example... If I were walking at 0.8 metres per second, I would travel 850 metres in...

Time = distance / speed

= 850 / 0.8

= 1062.5 seconds.