A recovery run is slower than your slow. If it feels like training, it isn't one.
The day after a hard session or a long run, you have three options: nothing, a normal run, or a recovery run. The middle one is the one to avoid.
What it is
Twenty to forty minutes, very slow — slower than your usual easy pace, slow enough that people who see you might wonder if you are all right. No hills you can avoid. No watch-checking.
The point is to move blood through tired legs without asking anything of them. You are not trying to get fitter on a recovery run. You are trying to arrive at tomorrow in better shape than you would have by sitting still.
Why not just rest?
Sometimes rest is right — see here. But gentle movement tends to leave legs feeling better next day than complete stillness does, and it keeps the habit of running intact, which is worth more than any single session.
The trap
Recovery runs are where the 80/20 split quietly dies. They start slow, you feel surprisingly good by minute fifteen, you drift a bit quicker, and you finish having done a mediocre easy run instead. Now you have a small cost and no benefit.
The fix is to give yourself a ceiling before you start — a heart rate you will not go above, or a pace you will not go under — and treat drifting past it as a failure, not as a good sign.
In practice, for us
The Sunday after a Saturday long run is the classic one. Thirty minutes round Seasons Park, talking the whole way. If you are alone and inclined to race yourself, take someone slower with you.