Right, exactly...I suspect that in Version 1.0, you won't choose to start a Fortress game as a separate thing from an adventurer game. Somehow, in some way, you'll just start a game.
If I may be allowed to raise a possibility for how this will occur, I would propose that one would start out as a dwarf in the Mountainhomes, and either set off on your own into the wild blue yonder, or try and raise supplies and volunteers for whatever crazy project you might have going (which might or might not be a fortress); so you could just leave the Mountainhome gates, OR you could go to the appropriate official and say "hey, I'd like 6 random ne'er-do-wells, some food, booze, and anvil and a pick and I'd like to settle here", or you could go to the appropriate official and say "hey, I'd like to borrow 5 of your best steel-clad soldiers to go attack that necromancer tower that raised those yaks two months ago." The "dorfing" system in succession games will finally be implemented, as you can switch in Fort "mode" between taking command of the expedition leader and designating things as God, and you can raise a family in a fort you can start from scratch. Two decades later, the once-turbulent outpost is a mighty tower, impervious to all would-be invaders, but your son has a hankering to bring glory to dwarfkind...and you can take command of your first dwarf's son and, I don't know, wander around drinking and partying and whoring in elven red-light districts.
This sounds a lot like the "embark scenarios" that Toady has mentioned a few times. My understanding is that doing something along these lines is an eventual goal, but that a simpler
embark now will be kept around if you just want to hurry up and play. There have also been musings on taking over existing sites from worldgen as another potential route to play.
All of this is probably fairly far off though...