The one thing splitting your routine does is to allow your metabolism that would typically slow down as the day progresses to rev up again.  That seems to help some people who struggle with weight loss or weight maintenance.  I'd also incorporate different routines like hills and valleys one day, straight longer run another day and entirely different exercise like biling or a stairmaster another day.  Finally, weight training can replace some of your aerobic activity in the sense that it will build muscle which increases your metabolic rate.  Two to three days a week of weight training with only an hour those days of aerobic activity could be a good move.