Well I wasn't there but I'll try and answer how I'd feel

First I'd be afraid that the southern tower will destroy the one I'm in as well. I would not expect a second plane because so far I wouldn't know it was an Islamic terrorist attack (if I didn't see a plane I would know it's an Islamic terrorist attack) and if I do I'd think a flight was taken hostage I wouldn't think two planes have been taken hostage

Second I'd run down the stairs avoiding the elevator like a good kid. Most people learned to take the stairs as fire safety training but for me where I come from I was taught to take the stairs because war was the norm and an explosion can destroy the elevator mechanism or put the power out.

Third I'd get out and run far away.

Fourth I'd check the news if I can.

Fifth I'd feel bad for the people in the tower.

All the above assuming I manage to get out and far.

Unrelated I would not expect the tower to fall like a crumbling biscuit remember when I said I'd fear the tower will take down the one I'm in with it? Because if be afraid the upper part would fall over. This does not mean I believe 9/11 was an inside job it just means I don't have the knowledge to guess how the building will fall. Yes, the way the buildings collapsed is not strange nor does it suggest an inside job.