ruined

Ruined is more like broken or destroyed. "The rain ruined my day at the beach."

So, is it meant although golf is a good way to walk, but it is destroyed by someone else?
The whole context is:

J: So what about you and your golf? Do you enjoy it?
D: To be honest, no, not really. I like the walking and the scenery and the company. I just don't like the bits in between when you have to hit the ball.
J: 'Golf is a good walk, ruined," as some one said.
D: W

So, is it meant although golf is a good way to walk, but it is destroyed by someone else? no

It means that the enjoyable walking and good exercise that golfing can provide is ruined or negated by the bad golfing results that most golfers usually achieve. Playing the game of golf badly ruins what would otherwise be a good and pleasant walk.