By running some of your mileage in the morning and the rest later on during the day, your body is getting most of the same physical endurance training benefits of a continuous long run since you're not completely recovering in between runs. However, the benefits are not as great as when you do one continuous long run. And, of course, it's definitely easier mentally to run 10 miles in the morning and 8 miles at night, rather than 18 miles all at once -- which is why you don't want to do it every week. Ideally, you want to run your long run continuously, since that's what you'll be doing in your marathon. But splitting your run is a great way to squeeze a long run into a busy week and it definitely beats the alternative of skipping your long run altogether.
Also see: Frequently-Asked Marathon Training Questions
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