hi leesha!
We did 13 hrs with DD when she was 6 months and then again when she was 9 months. (and 2 dogs lol).

i found it helped that she would take a cup as you didnt have to stop to feed her during the trip everytime.

Also we would put her in the car as soon as she woke up, so she was awake for about 2 hrs playing then sleep for a good 2-3hrs - it took a big chunk out of the day - rather than putting her in the car to drive when she was ready for her sleep.

Also stop at driver revivers to BF and graba toliet and coffee stop.

We had classical music playing in the back the whole trip (dangerous if it puts you to sleep though lol). and most of the trip one of us was in the back seat to read or settle her.
if we were both in the front she would get ancy!

make it fun and perhaps have an overnight stay at a b&b ... to break up the trip

but honestly have the car packed the night before, feed her as soon as she wakes then leave. it will give you a good start by the time she sleeps and wakes again!