Could I from the AD go to the terittory of one of the other factions?

From 1-50 you are limited to your factions zones, once level 50 you unlock the other faction zones, so once your 59 you can go pretty much everywhere, beat in mind not all locations will be available at launch, many will be added in further content updates.

Take a look at the interactive map
http://elderscrollsonline.com/en/map/tamriel

Thanks!

What about the landscapes in between the factions?
They seem massive! Will those be implemented too?

I think all that's available is those that are in the interactive map at this moment.

They will probably add more in expansions

Wauw!

Considering Cryptal (lol) is just as big as Skyrim, this means the overall map will be in one word: gigantic

They will definitely be constantly adding more and more, there's a ton of areas to add in content updates and expansion, furthermore I'm not 100% sure in the areas in the interactive map are all the zones at launch, there may be more they add to the interactive map later, but I'm not sure on this.

Seems like this will be the main game I'll be playing until I'm like an advlt or something

Yeh, from what I've seen/heard so far I can see myself playing long term, with regular content updates it should be a very fun and time consuming game

Ah true they did say the interactive map would still be updated, so it's even more