Some things just take way longer than we think they will. Sometimes it's sheer mis-estimation (setting up the basic CSS style sheet, getting the layout to look nice, getting the transitions working properly, etc. all seem simple but still require raw time and effort), and sometimes it's just that nasty bit that _should_ be easy and just ends up not being easy ("why isn't the DIV centering??") and you blink and 3 hours have gone by. And that part should have taken 30 seconds.
So it no longer surprises me when things that should be simple end up taking 10 or 100x as long.
BTW - fantastic article on this subject I read recently:
That all being said - OP - would love to hear about your process.