We flew 5 hrs at Xmas time with DS who was then 17 months. He was very excieted and a little hyperactive. He usually is at the best of times. We timed it so he would sleep for some it, which during those times was a relief, though god forbid it I needed to use the toilet. Luckily there was also enough space that we had the use of an extra seat for him to play on and on 1 leg has some 5-10 yr olds travelling behind us who was playing peekaboo with DS, that kept him entertained.
Good luck with your flight, it is hard work, just make sure you have some time to relax while on you holiday and a day or 2 after you return.