Different people recover in different ways. The first 2 marathons I ran I found the recovery quick and simple and I was back to running in no time. London I really struggled with. Like Max's Mum it was the legs.
I think that perhaps running on almost flat roads is what kills the legs. I trained on undulating roads as that's what there is around here so there's different pace and other muscles being used. I ran London at a consistant pace throughout.
I think it's a combination of impact and lack of variety in pace / terrain that kills your legs, esp if like me you are not used to this.
The lungs I don't know.
My only way around this was to run at different paces over different terrain. That may work for the lungs as well. I found that I found speed work easier than long steady runs. Getting back from illness and that's what I'm finding again.
I guess maybe it's being totally run down on energy. Maybe throw a couple of protein snacks into your diet. Quinoa is a very good food to eat.