Assumption :
Today's training run is 8miles easy run
Tomorrow's training run is also 8miles easy run
Scenario 1 : Run 8 miles (PM) today, run 8 miles (PM) tmr
Scenario 2: Run 8 miles (AM) and 8miles (PM) today so I get a rest day (tmr) without any running.
Which scenario will make you feel more rested? which will push u closer to injury?
Thanks.
If you're not used to doubling with that amount of volume you shouldn't be doing doubles.
If you want a day off but want to get in the volume and you don't have a long run, consider doing a 14-16 mile long run on day one then rest the next day, that's way more beneficial than either scenario.
To answer your question about the 2 scenarios, again scenario 2 has the biggest injury risk if you are not used to doubling with that volume.
100% scenario 1 is the better option. Scenario 2 is a super easy way to get injured, a friend of mine in high school literally got a stress fracture because he did that sort of thing (except it was two 10-milers). Rest days are nice, but it’s not necessary to have more than 24 hours of rest, and you should always start running 7 days a week well before you start doubling.
Essentially doubling exists to increase your workload while keeping it spread out (as opposed to adding the miles to a single run on that day). It’s basically a method of injury prevention for higher mileage (15 miles all at once every day is much riskier than 5 and 10 every day). However, your idea is the exact opposite of what a double is intended for. You’re actually shortening the recovery time between two runs, and even if you get more recovery between run 2 and your next run, it doesn’t change that fact that you’ve increased your chance of injury, for absolutely no benefit (possibly even a downgrade, because there’s some evidence that you need a night’s sleep in order to get the full benefit of a run or workout, and if you try to work out again before that you won’t get as much out of it).
There are so many variables to this that it's almost pointless to think about but here goes. If an eight mile run is so stressful for you that you need a lot of rest from it then you don't want to do two runs of such length in a day. If those are your only options go with eight each day. But if you want a fairly big day followed by a really easy one you might do something like eight and four one day and four the next.