I have flown alone with two kids and I survived just fine, though it wasn't an easy task. You will want the rearfacing seat by the window, older child's seat in the middle and you sit in the aisle. This will all change if you want both seats rearfacing as I would put the marathon rearfacing by the window and the infant seat in the middle since the marathon is bigger. Some flight attendants think that you can only have a carseat in the window position, but this isn't true. As long as you are traveling as a "family unit", you can have multiple seats in a row just so long as the carseats are not blocking your exit in case of an emergency.
There are some tricks for installing the marathon forward facing on the airplane seat. Which way do you plan to install it?
For carrying the seats through the airport, I used a luggage cart and used a bungy cord to hold the seat in place. I then put our carry-on luggage in the seating area of that carseat. I then turn the infant seat upside down and placed it over the first seat and carry-on luggage. I had my older child walk and my younger child was in a sling. When I went through security, I did have to unpack everything and send things through one at a time. When everything was through security, we just found a quiet corner and quickly assembled everything back again on the cart.
When getting everything into the plane, I lifted the luggage cart and all above the aisle and once at my seat, I disconnected everything and tried to get the seats installed as quickly as possible. While installing the seats, I just set the girls in some other airplane seats and tried to keep them occupied with the airplane magazine.
Here are some FAA documents you may want to print out and carry with you to indicate that you can install multiple seats in a row, and that you can rearface a carseat in the airplane.
Official FAA guidelines on carseats on planes
New FAA clarification memo on carseats on planes