I think it might be difficult, and nearly impossible, but do not dispair!
I would ask your hotel; they will be in a better position to tell you how to do it; it is surely not the first time it happens.
Rome Marathon information
The start time is at 8h40am (up to 8h52-ish). The best runners will arrive around 10h50am; the bulk of the runners will be arriving for at least the next hours up to 13h40-ish (4-5 hours marathon).
The marathon officially lasts 7h30; the last road to open will be near your hotel.
Your plane lands at 9h45, it takes about 1 hour to get your belongings, and takes around 45 minutes to get to Termini train station. I imagine that if you take a taxi it will be easier to drop you there); so you will be "near" your hotel at around 11h30am. You will have to walk to your hotel.
Normally at events like that, there are officials posted at street intersections, police persons, voluntary persons... I would seek one out and ask them if it is possible to cross the street, usually with the assistance of the officials.
Good luck, if all fails, just find yourself (beforehand) a good cafe or bar and get your first coffee or Spritz and just chill.