I ran 17.5 Sunday in temperatures starting just a few degrees above zero degrees F.  But the temperatures were not an issue.  And neither was the distance.  I find when I am in decent shape the difference between running 5, 10, 15 and, yes, 17.5 miles isn't much (except more time on the road), so long as I am just doing a not-to-hard training run.  I focus on the running, but not the accumulated exercise, if that makes any sense.  Runner's zen, perhaps.  Like I say, just turn off your brain and put one foot in front of the other.  Except that you really don't turn off your brain, you just set aside thoughts that dwell on any growing tiredness and any increased discomfort and instead, well, just keep on running, enjoying the miles and the day.  It works for me.