I'm in day 14, and my energy isn't completely back, but it is much better than day 4. A lot of it is mental, although it seems physical. Keep going on walks; take longer ones as you progress. On day 7 I went out and did yard work, cleaned the pool, etc. I DIDN'T LIKE IT, but I did it, and when I was done I experienced a sense of satisfaction for the first time in a long while.
As Sarah said, we're all different; you may be running a marathon in a couple of days, who knows. Day 4 was the turing point for me. Keep doing what you're doing. Keep reading and posting.
It varies with everyone. Sometimes it can last a few months. You are doing great on day 4 so keep up the walks and the supplements!