Clarification: I can choose to do a task, including Fortress Mode tasks, regardless of my profession, and all of my actions are governed by the DF mechanics, right? And all the bugs are carried over from this version?
If so, I'll exploit the hell out of that world.
First things first: when I spawn in a village, I find the nearest abandoned building, dismantle 10 tiles of it for wood, construct a Carpenter's Workshop(1 wood) and a Bowyer's Workshop (1 wood) and a cheap 2 Z-level-high one-training-spear Shaft of Enlightenment(3 wood), then build a shield, a training sword, a training spear, a training axe and a crossbow(5 wood).
Then I wield them all and throw myself off the Shaft. Two possibilities as to what happens when I hit the ground: 1) I get hit in the head and die; 2) The Shaft breaks a few non-vital parts of my body but I live.
Possibility 1 ends with me dying within a few hours from spawning.
If 2 happens instead, I'll sleep for an hour, which tends to cure broken bones for some reason, then rush to a nearby keep to get some stone blocks and build a soap production line. If I'm quick and lucky enough, I'll have cleaned whatever smashed nails I have before the infection sets in; as a result, I'll get to live for a few years or decades as a fat, arthritic, near-invulnerable Enlightened adventurer. If I don't succeed in preventing the infection, then I'm going to be dead in a few months.
That said, if that exploit didn't work, I'd probably die really quickly. Weak people don't last long in DF.
Conclusion: my expected lifespan could be anywhere from hours to decades.