Although we had absolutely perfect conditions, our day three weeks ago on Ixta was pretty tough: http://www.splattski.com/2008/mexico/day_3.html
That said, there are a lot of variables in answering the question for yourself:
You're fit... but for what? Are you comfortable walking up steep hills in loose terrain?
If you are doing it in a day, are you trained to be on your feet for say, 12 hours?
What elevation are you acclimated to? Staying at Amecameca is a good first step, but it's only at 8k. If you are spending the night at the hut, you'll be sleeping at 15k, higher than any peak in the lower 48. If you read through summitpost articles, you'll find plenty of people who had altitude headaches at much lower elevations.... and it's about 3-4 hours (or more) from the hut to the summit.
ON the other hand, Ixta is not technical. So if you are willing to put forth some effort, it is doable for someone without any other peak experience. Your guide should help you take care of yourself to deal with the altitude as best possible: pacing, hydrating, eating, etc.