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Several marathon training plans that I have read would have me running a gentle 5 mile run the day after my weekly long (15-20 mile) run.
My uneducated opinion is that should be the most important rest day of the week. 5 miles is certainly not a massive challenge but with undoubtably sore and tired legs after a lengthy run it does seem like a tall ask.
What is the thinking behind doing this?
The marathon training program I'm following has 5 workouts/week. If you do the Math, you'll see there's quite a lack of rest days. This program consists of the following runs: 30 mins, 2+ hr, 30 mins, 30 mins, rest, interval, rest.
The need for rest days depends on the intensity of the runs, since in my case the 2+ hr run had to be performed at a relaxed pace. It was more important I ran for a long time, than that I tried to run fast. While your legs may feel heavy and sore the day after a long run, generally a sort relaxed run will actually make your legs feel better. This is most likely due to the increased blood flow, which helps get rid of any residues from the long run and supply your muscles with a healthy dose of blood, which can aid with the recovery.
In my personal experience I always felt more sore after the interval training, than after the long run. Muscles get damaged more during intensive workouts, partially because the impacts are (much) higher and your muscles will need to generate a lot more energy in order to achieve higher speeds.
You could switch your rest day to right after the long run, rather than before the interval training, but I think taking rest before the interval will help you perform that workout better. That way your muscles get the time to stack up on glycogen which you'll need for the interval.
So no, you don't need a rest day right after a long run. But your mileage with my answer will vary with the intensity level of that run. If its a race, yes you will need some rest.
Many marathon programmes will have such a run as a recovery run as many people finds it helps.
For ultra-runners, you'll often find that they do back-to-back long runs. Big long run on one day, big long run the next day. This helps get used to running on tired legs and increases the endurance without having to do one huge long run. (Trust me, doing a 35km followed by a 45km makes you very tired).
My coach would say that the recovery run is optional - it's about recovery so you have to see how your legs feel and listen to your body to decide to do it. Certainly, the more you run, without injury, the faster you'll be.
Lastly, lactic acid. Yes, most people blame muscle soreness on lactic acid. As far as we know, lactic acid has nothing to do with it. Muscle soreness is because there are tiny micro-tears to the muscle fibres. Your body will come back and repair those tears but make them a little stronger - this is what makes you better.
In general I think it is to help force some of the lactic acid out of the muscles. Just take an easy run and you will recover faster than a total rest day. The other argument can be that taking a light jog the day after a hard run can keep your body from peaking. If your body decides that it has peaked before you actually race, your race performace can be worse than your last good long run.
