Training for long distance walking

Id say walking lol

start with what you are comfortale doing and gradually up the distance, once you have got a good distance you are comfortable with then maybe look at your pace (if you would be againts the clock on your LDW) or stay at a steady pace you are happy with.

try and keep a posture (spelling?) while you walk too as it will be easier on your back and joints etc if you are walking well

Invest in quality footwear & stock up on grey wool socks (yeah, I'm old school, I love wool....) and get out and walk in conditions that mimic what you'll be doing as closely as possible.

If you're not accustomed to walking longer distances be sensible and build up gradually. Once you've built up your stamina walking without a pack start walking with a pack simulating the weight you expect to be carrying.

Like any other fitness type of activity cross training would be beneficial (especially core & upper body as, presumably, you'll be carrying a load)

I did Nijmegan while I was stationed in Germany in the army a while back. I'm not sure how far you are planning to go and how much daily. Nijmegan was 26 miles per day for 4 days with some minor rolling hills and cobblestone roads in places in military gear, boots an a 40lb rucksack. we walked anywhere from 5-20 miles a day about 4 days a week for almost 6 months in preparation. You will get blisters on long distances and learning how to prepare how to handle those will be one of the biggest issues, chafing may be another. I found my feet swole up enough to go up half size in boots after the 2nd day. Ice and elevate your feet afterwards to help keep swelling down, invest in some anti-inflammatory but for now, go walk.........a LOT.