It would be great if you could do an as honest writeup after some time(possible a couple of months). I would be interested in thoughts along two lines:
1. Maybe working Google will not be that great. They sure do have very smart ppl - but I guess you know by now that startups are a different beast. There was a reason you went into the startup world - maybe because you found more meaning in the complete responsibility a startup endows you, and probably that wont exist in Google - nor the adrenaline. Maybe you need to balance that 'meaning' in your job with having fun - your starup was all about meaning and probably too hard to be fun so a stint at Google looks exciting. Maybe you will crave meaning very soon, in which case Google might not be the best option.
2. "And I have no cofounder, so I'd be doing everything myself until I could afford employees, and then I'd have to build a company culture. That will be no 'fun'" Maybe you can still find a cofounder(maybe in Google). I am not sure, but I think there should be a way to do startups while keeping the fun intact. One thing that I have seen is that big companies afford you a default(big) social circle which a lot of times is the source of the fun you have. (Also startups typicall dont have cute HR's ;)). But maybe you can still create a big enough social group by just hanging out with other startup folks and have as much fun.
Just some random thoughts. Feel free to (not) reply.