Such an individual question, and a lot of it depends on your existing fitness and choice of route. I'm a believer in the hike your own hike mantra, some people prefer gentle strolls through glens appreciating the wildlife, whilst others are ploughing up peaks until they collapse on their sleeping mat - and of course everything in between.

In general for LDW I'd say cardiovascular fitness is useful for challenging routes, yet the most useful I've found is mountain running over uneven ground, letting your ankles and knees strengthen.

Personally, my prep is more individual fitness, I do strength training 3x weekly, row daily, and we will do a couple of weekends away prior with full gear to test our kit choices and remove whatever is an excess.

I'd say the most useful prep is getting yourself out on overcast, rainy days and putting some serious miles under your boots. Everyone understandably tends to gravitate to the nicer days, and the psychology of being out in the middle of rain, high wind whilst still navigating and feeling warm and comfortable will make it easier to do later.