What the benefit of two-a-day workouts?

It is to get in more miles. In my opinion, any miles that are run at recovery pace or faster will give you more fitness.

Have there been any studies showing that the extra recovery-pace mileage is actually beneficial?

Same benefit as any run except you get twice as many.

More recovery days.

Let's say you do two hard days a week.

And, you have about 3 days or so to recover.

Well, with delay muscle onset soreness you won't feel it till the next day.

Another advantage is to build up endurance if you are peaking for a championship meet with several heats/if you are doubling at a meet.

Another advantage is more volume at race pace.

ib_runner wrote:

When I look at training plans that have two-a-day workouts, I notice that the second workout of the day is an easy few miles. What's the benefit of this? It seems like just junk miles to me. Is to just to aid recovery or does it serve a purpose in building cardiovascular fitness? capillary growth? other?

Thanks in advance!

Your question is entitled. There is absolutely no scientific evidence that the extra junk mileage will give you an extra boost that you won`t get in the single run. In fact the single run, if it`s enough far and at best individual pace, will give you the aerob power you need to complete your quality running.

At some point if you're doing big mileage, doing it all in one run has its drawbacks and breaking it into two runs makes the whole business more manageable. I found that I could do 90 miles a week in one session easily and even 100-105 was pretty manageable. But once I got 115 and beyond it was bear doing at least a 17 mile run every day. And to run that far in one shot you need to keep the pace kind of slow. Doing two runs made it possible to go a bit faster with no greater effort.

Just curious- did you ever try keeping all your quality mileage the same but just reducing the recovery mileage so that you could stay at one-a-days and total maliage per week remained at 90-100 instead of 120+?

Running faster

Anecdotal (although there may be some science on it), but in the training periods that I doubled, I felt like I recovered faster.

As others will likely point out, if you are running high mileage (say 90mpw+), doubles become essential because you don't want to run 10+ miles on every single run.

"Junk miles" is a term that gets thrown around without a lot of context. Any running you do is going to be beneficial, even "slow" running. In the context of any good training plan, you will have hard running and easy running. The easy running serves a number of valuable purposes and is not "junk."

ib_runner wrote:

Just curious- did you ever try keeping all your quality mileage the same but just reducing the recovery mileage so that you could stay at one-a-days and total maliage per week remained at 90-100 instead of 120+?

I just ran as i felt. I never really used the concept of recovery days and I considered all running as "quality running.." If I felt like running hard I ran hard and if I was too tired to run hard or moderate I ran slowly. I did alternate between times when I was mostly in the 120-150 range and times when I was in the 100-110 range. Usually when I was in the lower range I was running faster than when I was in the higher range.

Two-a-day workouts are just there when it doesn't make sense to do more miles in a single run. I like to run a little shakeout a few hours after my workouts to flush lactic acid and get a couple more miles on the week. Flushing the lactic acid has helped me with feeling fresh for my next run, and it's always nice to make that weekly mileage total a little higher

Sand Dunes wrote:

More recovery days.

Let's say you do two hard days a week.

And, you have about 3 days or so to recover.

Well, with delay muscle onset soreness you won't feel it till the next day.

Another advantage is to build up endurance if you are peaking for a championship meet with several heats/if you are doubling at a meet.

Another advantage is more volume at race pace.

I'm sorry, I thought you were asking about doing two workouts a day. Not doubles.