I'm running about 80 miles in singles and about 20-25 miles in doubles. Currently I'm doing 4 doubles each six miles, but I'm curious to find out if there is any major difference between that and doing 3 doubles each eight miles, still 24 miles. Assume that all runs will be run at the same pace, so don't say "you will run faster if you do 4 doubles". Running 3 doubles will save me some time but I'm just trying to become a better runner so I'm fine with 4 doubles too.
A mile is a mile. Theres plenty in The Nile.
- Malmo
do 4 doubles of eight miles each
Asking for the difference between doing 3 doubles/week versus 4 doubles is one of those questions you aren't going to get a clear answer on. There are a few benefits/negatives to look at, but I feel that either way you do it, it won't make a large difference on performance at the end of the summer.
Some things to note about 3 versus 4 doubles:
Doing 4 doubles is mentally tougher becuase that's 1 more day you have to get up early to run twice. However, it allows you a little more rest between miles. You can run in shorter spurts because you are doing it in 4 sessions versus 3. This is slightly less stress on your body. Think about the difference between cruise intervals and a straight through tempo run. The difference is that cruise intervals allow you to run slightly longer/farther than 1 straight tempo because the run is broken up and allows your body a slight break from the stress. I realize you want to stay at 24 miles no matter what but it still breaks it up for you and reduces the stress level, somewhat.
*On a sidenote, it's not always about the quantity (miles), but rather, about the quality (time) spent on your runs. There should always be a purpose - you should never go running just to add another 2.5 miles to your total so you have exactly 100 for the week. There should always be a REASON and goal for your run (recovery, aerobic, anaerobic, speed, LT, strength, etc). This is why I suggest switching it up depending on how your body feels that week. If you are feeling tired and beat up, do 4 doubles of 6 miles each to split it up, or just do 3 doubles of 6 miles each and don't force yourself to do the extra 4 simply to have "x" miles for the week. Garbage mileage isn't beneficial in the end if it hurts your performance. If you are feeling strong, keep a good recovery pace but maybe do the 3 doubles of 8 miles each for a little more of an aerobic effort.
Just my experience, but 6 miles is the limit for me if I need to do a quality workout that same day. Actually, I like the AM run to be about 5 miles. So one difference might be that it's too much to be fresh for the PM workout.
Always ask yourself: what is the purpose of every run?
The purpose of the AM run is to get your body awake and burning calories, loosening up your legs, speeding up your recovery of the previous workouts, training your body to recover faster, and get some aerobic training in WHILE still being easy enough to recover from/get ready for the hard workouts. Sooo..... If that's the purpose, doing more doubles is better, even if they are very short. 8 miles seems too long to be considered an add-on, but I suppose it also depends on where you are in the season. If you are just base-training or increasing miles, the 3 doubles would be OK.