Working at a startup is a great way to get some experience in several other domains, especially if you are one of the first employees. Even if you don't do it first hand, you'll get to spend a ton of time with "head of sales", "head of marketing", "head of product", because they'll be the other three dudes in your office.
Get used to the pace, risk, and general mindset of a startup first. There is a nontrivial adjustment period between working for big companies and working for startups. So I would highly advise you to go through that without the additional stress of running the startup and working on it.
With a startup (or more specifically, your product), timing can make or break you. Releasing a tablet in the '90s was too early, because there wasn't freely available WiFi or mobile broadband. There were electric cars before Tesla, but they didn't take off because battery technology wasn't good enough, and gas was too cheap.
It is a good question to ask yourself (because the VCs will). Why is now the perfect time to launch this startup, and not 2 years ago or 2 years from now?