Trenitalia Train in Leonardo da Vinci–Fiumicino (FCO) Airport

You can walk to the train station at FCO. The real issue is immigration control. This timeframe looks a bit tight to me.

We caught this train after and early morning arrival last March. The FCO train station is just opposite the exit doors. There are plenty of signs or someone can steer you in the right direction. From terminal 3 you will take a shuttle train to customs. We have typically gotten through customs in about an hour, but it can take longer. Luckily, there is a cafe near the train tracks so you can sit and have a coffee or snack if you have extra time.

Get the Trenitalia app; buy tickets after you get your bags.

It can vary based on staffing and how many flights arrive at once. We flew into FCO this summer and we got through immigration in under 10 minutes.

If you have any airline delay, you could miss the train and lose your train ticket, I agree with previous person, buy once you arrive.

Do you pass through another European airport before arrival in Rome? If the airport is part of the Schengen area like Munich for example you have gone through the immigration control and can proceed quickly through the nothing to declare line . So minimal time.
If direct into Rome then more time is needed for customs and immigration.
There are only a few trains to Florence that depart directly from FCO airport. The 11:08 departure appears to be one of those.
The trains go from the main FCO train station. If your plane is on time and I assume your train is direct from FCO to Florence, I would say your chances are good.

I've taken that train multiple times with a 9:15 arrival and generally had a long wait for the 11:08 train to depart. The one time my flight arrived quite late I still made the train, but just barely. If you have checked luggage, then the timing will be tighter.

A note to other posters, the OP has already purchased tickets for the 11:08 train.

Thank you to everyone for your quick responses! I appreciate your feedback.