After my long Sunday run I did 45 min on the bike yesterday and an easy 6 mile jog with the dog today- I need at least 2 days to recover, and hopefully will be ok to bounce back with a hard interval session tomorrow.
Sometimes there is a fine line between needing to put in the distance and requiring too much time to recover. Kenny Moore, who placed 4th in the 72 Olympic Marathon, used to do his long ones every 10 days as opposed to every week. He was logging up to 30 miles on those long ones so needed the extra days.
Some believe that you can train very effectively for a marathon by running just 3 days a week, one long, one medium, and one interval/tempo run. Not a bad idea, especially as you approach 20 miles on that long run. I probably did not need to run today either, but whether or not an easy 6 recovery run does any additional harm is anybody's guess.