Yup, probably dangling legs! You could try some of the things that the ladies above have mentioned, and it should really help the problem. Another suggestion would be to pick up a foam beverage cooler (emphasis on foam! it must be very light and not hard plastic, as this presents a risk as a projectile), put some duct tape around the edges and top to keep it from getting destroyed by shoes, and let her rest her feet on it
If this doesn't help, you can try putting the Marathon in its second recline position (only recline positions 1 and 2 are permitted for forward facing use, recline position 3 is for rear facing only, but this becomes quite clear when you have it in that position!), this may help tilt her backward more and keep her bum back and scoot her legs in a little more.
Also, I know you have checked the seat for anything that may be sticking into her back, but have you checked that the black foam pad located along the back of the seat where her spine would rest is not rolled up and is lying flat? I have raised and lowered the harness on my Britax seats and had that piece bunch up. I have actually taken the comfort foam out of the back of my seat as the kids prefer it that way :scratcheshead:
HTH!