My uninformed opinion says never. Running 6 miles twice is great but it’s not the same as running 12 once. If you must, you will still get good workouts in but not the effect you are going for with a long run

When my coach usually gives me double runs it’s to split up a long run into a longish run and a shorter run. So if I’m doing an IM or marathon I might do like 16 miles then 4-5 later in the day. But I’ve also done 2 like 5-6 mile runs in a day but not to replace a long run.

---

Twitter - Instagram

https://forum.slowtwitch.com/forum/?post=4991789

It’s really all about accumulated fatigue.

Say I have a long run of 15 miles and I know that at my training pace it will take me 2.5-3 hours to cover that distance. (Yes, some of us really do train that slow.) A 2.5-3 hr run is a lot of stress on the legs… Depending on your conditioning and how fast you recover, that 15 miles could affect the quality of your workouts in the coming days.

Splitting the run allows me the benefit of the long run, lessens the possibility of injury, and promotes a quicker recovery.

How you split the run is up to you, and probably doesn’t make a tremendous difference. For the 15 you could do 6.5 in the AM, have a short 2-3 hr recovery period, and then do the other 6.5… Or you could do 10, then 5 after the break. All-in-all you’re still doing the 15…

I get that mentally I’m going to feel more ‘accomplished’ in doing a straight long run, but let your body tell you what’s best. Not your ego…

I disagree completely. All miles are not equal. The physiological stimulus provided by one 18miler in the morning is absolutely not the same as a 9 miler in the morning and one in the evening. I could do the former for maybe 3-4 days straight but the latter almost indefinitely.