For a big trip, probably the first thing I do is check out the guidebook options and buy a good map.

For an extended trip, I would put packing earlier. I leave in a bit over two weeks for a trip but I am using a bike I have not toured on before so yesterday I was test riding that bike with the racks I will be using with most of the gear already loaded in panniers to see how it handles. I have to worry about weight for flying, so that is also a reason to pack early. Today I expect to put more weight on the bike to simulate the additional weight of fluids and food. And if I have time today I plan to try it on some hills, both up and down. If you have a bit of hardware that is not quite what you need, you want to learn that early and not be rushed to decide what to buy on really short notice. Several years ago I received my rear rack in the mail the day before I left on a trip. I could have used the heavier and less stiff rack that I had toured with before, so if the mail was late it would not have been a disaster. But I still do not want to cut things that close again.

Maybe add check on cell phone plans where you are going? If you want any maps on your smartphone, add downloading those to your schedule. I like to have any weather forecast, weather radar, lodging, and transportation websites identified long before I leave so I can have those on a tablet or phone. I might also want to save some other documents on my phone for viewing during the trip.

Financials to include credit cards, debit cards, ATM cards, arrange for how payments will be made, notify financial institutions, etc. And backup plans for when something goes wrong. I know someone that had to sit in eastern Europe for several days while waiting for his new credit cards to arrive before he could leave and continue his trip.