I did my play through in one sitting so I was left with a lot of distance to cover.
Obviously this should be the last thing you go for as you will accumulate distance for everything else you do.
In the end the method that worked best for me was power plant. I just walked around the big circle path near the dumper trucks.
Pro tip - plug in a second controller and walk round with both. Distance will go up twice as quick.
There are other methods you could use such as riding the dumper, speedboat or tanker ship. Use which ever you feel best, for me it was just walking round.