Having lived in the flats of Texas my whole life...and gone to Colorado and the mountains therein 100's of times if not in the 1000's....I would say not to worry too much about altitude sickness. Just know that it can happen and depending on the severity what you should do about it. Otherwise quit worrying about it. You are not attempting to summit Everest!!!
I have never had it that I know of. Never seen anyone I was with have anything more that a regular headache that some ibuprofen can't handle. Drink lots of water. And drink lots of beer and whiskey if it suits ya!!! We do. I have been up when I was in pretty good condition and when I was a fat slob, and walked until I got tired or was where I wanted to be. Never felt ill of it. Only difference I can see for those of us from the warmer regions. Low oxygen means you have to stop frequently on uphill treks to catch your breath. But the fact that it's not 105 degrees means a fat man can walk all day if he paces it right! Always a trade off.

So unless you have health issues of some kind, quit worrying about stuff like altitude sickness and worry about the important things....like how the hell am I gonna get this big muther out of here now that I killed it!!! Good luck and have fun.