Sourdough Starter Very Runny
Actually, reading further on the link you provided I found this:

Why does this starter begin with whole-grain flour? Because the wild yeast that gives sourdough starter its life is more likely to be found in the flora- and fauna-rich environment of a whole-grain flour than in all-purpose flour. What if all you have is all-purpose flour, no whole wheat? Go ahead and use all-purpose; you may find the starter simply takes a little longer to get going. Also, if you feed your starter on a long-term basis with anything other than the all-purpose flour called for here, it will probably look different (thicker or thinner, a different color) and act differently as well. Not to say you can't feed your starter with alternate flours; just that the results may not be what you expect.

So perhaps your starter is just thinner because you're using AP flour. If you can, try to add rye or whole wheat to your feedings. That should make the starter stronger.
Another thing I'd try is to wait longer between feeding and check the starter's behaviour: how long does it take to rise? How much does it rise? Start feeding after the peak (when it starts going down) and see if that changes the behaviour. If you see hooch it's past feeding time, try to feed your starter sooner.
You can also try using less water (instead of 100% hydration lower it to 80%) maybe there's not enough gluten to keep a strong structure at a higher hydration level.
In any case, make sure you use water that is warm enough (24-30ºC) and be patient! It might take longer to get your starter stronger (it took me 2-3 weeks the first time, with the wrong kind of flour).