YES! I can totally relate.
Both times I've trained hard to do an iron distance triathlon, I felt that way for a couple years afterward. The first time I didn't race for 3 years afterward, this time I haven't raced at all this year. It's like "ok, I did that- what's next?"

I haven't felt that way about my career, yet I feel that way about owning our own house. We saved, we bought, we almost have it paid off, and now I want to sell it and start over someplace new (in a much smaller house on much more land).

Nothing is wrong with you, I think that sometimes we set such huge goals that working towards them is our only focus. When you finally achieve that, you have nothing left to work towards so you need to find a new challenge that inspires you. You just have to look and find out what that will be.
I have to figure that out for myself...