I have taken dd1 on long flights a few times.
first one was when we came back from living abroad when she was 5 months old - that was a 14 hour flight.
since then we have been on short flights a few times - Scotland, Portugal, Germany.
and also been to California (3 years ago, when dd1 was 3) and Australia (18 months ago, when dd1 was 5).
we are going to Florida (Disney!) at Easter.
I would certainly say if at all possible take at least one short flight beofrehand. I have found it is not so much the actual flight that is the problem - you can plan for distractions and entertainment, and food - but the check in, security, waiting at the gate, and then getting off, long walk to baggage reclaim etc that is the problem.
now that dd1 knows what ot expect (and that dh and I have sorted our routine out - this is a simportant, imo!) wrt who does which bit of childcare/checking in/fetching trolleys etc, and now e have refined and sorted out our cabin bags so we know what gets packed where, it all goes well.
the flight has never really been a problem. Claifornia flight was a hurdle - but due ot dietary issues. dd1 woudl only drink one type of juice form the carton, and obviolsy with restricitons etc it was a logistical nightmare. dd2 was 14 motnhs old at the time as well so was a handful too. but we lived. in total, dd1 was probably noisy ofr an hour tops. she spent a lot of time singing (nicely) and reciting stories, which annoyed other passengers, but what were we supposed ot do? gag her?
the hardest part has always been security - walking through the arches one by one, separating yourself from an upset child is not easy, and also putting eg favourite comfort toy thorugh the x ray.
my tips ofr the flight itself: ipod touch/ipad for dvds (has a better battery life than aportable dvd player). sticker books. the biggest hit with dd2 one year was a set of bath toy dinosaurs (brought along ot take into the swiming pool at the other end!) - she played for ages with those.
would you need ot organise food too? we have to, as gf/cf, and that can be a pain, but also do-able. shout if you want any pointers on that.
oh, the other thing - when dd1 was 4/5 we managed to persuade the stewards to still give us a baby belt for her for take off/landing. they didn't like it, but our reaosning was she would be as safe with that as she would in her seat - not sitting up properly, wriggling about, loosening her belt, wanting ot sit with me etc. and they agreed. tha tmade it much easier, as the big problems on the flight are those times - noisy/have ot sit down away form me etc.