In reply to Wide_Mouth_Frog:
As mik82 says, you can recover a working starter from something surprisingly manky-looking. If it’s got black liquid sitting on top, just dig under for a teaspoon full of the putty-like paste underneath.
If speed is important though, you’re better with yeast. A sourdough loaf can take all day (though you don’t have to do much). It’s the sitting around gently fermenting that gives the extra flavour.
It took me a few goes before I was happy with it, but it is great once you get the hang. I found Dan Lepard’s book helpful (and inspiring) too.