You're going to get drastically varying opinions on this here. It's a matter of preference and personal belief, to a large extent.
That being said a 10 mile run is more beneficial to your body than two five mile runs separated by eight hours, this is logic, not personal belief. Of course there are a variety of reasons to cover ten miles on your feet and simply gaining the greatest 'workout' is not necessarily the one you're looking for.
If you're just looking to build fitness...as you presumably are during the summer...yeah, a single run is ideal. Of course, it depends what your mileage has been before...if you have never run over 40 mpw I wouldn't suggest going to 70 in singles right away. Secondary runs can be a good way to ease into the mileage...for example doing 3+7 a few times a week to get you 10, and gradually changing that to a single 10 miler.
Again...people are going to disagree with all of that. It's a matter of opinion..some people believe that it's ideal to run two or even three times daily.