there are articles out there about running "doubles". not sure where they are, but do recall discussion about them recently.
i used to run doubles all the time. 2 miles in the morning, 2 in the evening. when my body felt up to it, i did triple runs, but never over 2 miles on each run. i started doubles pretty early in my "training" and don't recall having any issues. i just listened to my body when it said "not today, please".
the information i am seeing is that in short distances, doubles can actually be very beneficial. in long distances it is less beneficial and more prone to injury as the muscles don't have proper time to recover after the longer runs. not sure how long their "longer" distance covers, so you might want to check that out.
I would say, if you listen to your body and don't overdo it, doubles shouldn't be a bad idea.
love & junk
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