Don't tell them. Their goal at 1st level is almost never to take down the BBEG, it's to take on the smaller, closer, more managable, but still immediate threat. That threat will lead to other threats, other challenges. Though YOUR ultimate goal will be to have them take down the BBEG that doesn't have to be the PC's goal from start to finish.
This gives you the chance to build the BBEG into a major threat even if starting from obscurity. It lets the PC's fight OTHER threats without feeling like they're abandoning the overall goal of the campaign. It lets in the possibility of the campaign taking it's OWN course rather than the ONE TRUE course that you insist it follow without deviation.
I too have run campaigns where the PC's start unravelling mysteries and conspiracies and in discussing their own theories come up with great ideas - better than my own. In one such campaign I never did have my own overall plan - but I suggested that there was one. The players then took otherwise unrelated details that I threw at them on a whim and came up with their own conclusions that SET the whole course of the campaign without them even knowing it.
In short, the buildup of the BULK of the campaign has to have meaning to the PC's aside from being tedious preparation for the REAL fight that they won't get to for months or possibly years of real time.
