This is a summary of my Sudreyar VH campaign getting "War Without a Breath" achievement.
The first 40-50 turns were really just to get the feeling of the campaign, the geography of the area and the strength of my troops. I quickly realized that my capital, while being on an island, is not really far from anything... Fleets come from both the south and the north. Orkneyar islands are actually much closer than the inland, so I decided to try and expand there. I made a few attempts to push south, but as soon as my armies sailed - Orkneyar came from the north. So, they had to be weakened, or eliminated.
The same problem - once I moved north, invaders came from the south! However, this time I followed through with raids into villages further east, searching for food and sack money.
In the early days, until turn 60 or so, food was my main issue. Getting the production high enough, so I could field a good stack, and actually win battles, was my main goal. I had two fish villages near me, which was good to start with, but long term I needed much more food. Very quickly I realized I must always keep a reasonable force at home to defend against attacks, and this limited my ability to expand.
The turning point came when I began taking advantage of the strength of the province capitals. I risked leaving my back exposed, to capture some capitals. Then I raced back to defend home, leaving a smaller force in the new garrison until it was replenished. Repeat a few times until I got a good base in the northeast.
I thought I was doing fine, but then the Northumbrian Vikings landed 5 full stacks on my doorsteps! That was a mighty challenge, trick them into laying siege then killing their stack, then retreating from a village and trapping another stack in it, etc. I lost my home province for a short while, but by then I had 3-4 other provinces so it was fine, and if I didn't stop the invasion I would lose anyway.
After that it was a constant trench war going down south, slowly but surely. I had a little scare at one point, when all of England united against me, into a single country - England! However they were not very strong and I defeated their armies quite easily. At that point I was just going for all the victory conditions.
Strangely, when I got to 70+ settlements, I started losing income and food for each one I captured! This was due to corruption... So I halted a bit to rearrange the economy, reduce corruption, appoint governors, increase food production etc.
I got the "War without a breath" at turn 128, then Viking invasion came around turn 197, but it was merely a nuisance. 7 stacks were able to take a bunch of settlements, but my armies went back north to eliminate them quite quickly. Eliminating the last southern fleet took longer, as it was behind enemy lines... so I just sent my armies over there, not trying to hold anything, just hitting and smashing, in a true Viking fashion!
To summarize, the beginning was hard, much harder than I thought, despite the isolation I was enjoying. A more central power like Mierce or Mede could prove a bigger challenge, unless their initial economy gives them a better boost... We'll see!