Thank you guys for your thoughts.
We're both debt free, though I'm a bit behind on retirement, have no plans on buying a house and we're both youngish (well he moreso than I, 26 and 31 respectively) and leaving the military life in Japan so it is the perfect timing.
Now I'm worried about the trip budget as many have mentioned $20K won't suffice for 2 people. (It doesn't include transport though). We were going to do 4 months in Thailand, India, Nepal and then 5 months in Greece, Bosnia & Herz, Croatia, Italy and France (doing the GR and perhaps a lot of camping) Is this just not feasible for two people? We were thinking $50/day Asia and $100 day/Europe. (again, not including transport)
So to clarify, is the $20k figure just for on-the-ground costs like accommodation, food, activities, local transport? I.e. not including international flights, travel insurance, visas, vaccinations? That sounds enough to me. If it includes the other stuff, it could still be enough, just that you'd have to be more frugal and resourceful.
I can only speak for Asia (my partner and I have spent a month in Nepal as well as a month in Thailand; and my partner also spent a month in India with a sibling). All the three countries you are planning to go to can be very cheap, if you're not very particular about things and have simple needs. If you'd like to know a bit more you can have a look at page 3 of my journal for info about my Nepal trip, and page 9 - 10 about my Thailand trip (though we only explored North Thailand that time). I have travel posts in my blog (in my signature) as well. Feel free to ask any questions. My partner and I spent a straight 4.5 months in Asia and had an awesome time. In Thailand we ended up spending about USD$40 per day for the both of us (not including return flights to Thailand, travel insurance, vaccinations, and didn't need a visa), and that's moving around a fair bit. If we had just stayed in one spot we could have gotten a discount on accommodation and scooter hire, and spent even less. Nepal is also super affordable, but the costs can go up if you do things like a trek or a safari (paying for the overall thing, plus individually tipping the guides, porters, drivers, etc.).