I’ve walked from Fort William to Cape Wrath (my route was about 370km/230 miles) over a couple of weeks (with 2 rest days) , and we aimed to average around 20-25km a day (which in hindsight was a good decision), with a couple of monster 30Km+ days thrown in. The route was planned to be as “remote” as possible, so very little road walking and lots of ascent and rough/trackless sections. At that time, my “walking fitness” was very good and I’d have struggled to sustain a higher daily distance than we did over that period of time – so if you’re not doing long walks regularly, you may find it difficult to sustain 20+miles/30+km/day without various bits of your body complaining. I walked the West Highland Way in 3.5 days a few years back (with a certain Mr. Druidh in fact 😉 ) and that involved that kind of daily distance on perfect tracks and the wear and tear on your body is significantly more – it also magnifies any small problem with the fit of your boots/socks/rucksac, etc. Shorter days have the benefit of getting a decent rest at the end of the day – if you finish late and start early, you’ll soon wear yourself out.
Druidh is spot on – day 2 or 3 is often the killer on longer walks – once you get past that, you start to get into the swing of things. I’ve never travelled super-light, but any weight off your back is good obviously. I found the main thing was eating enough of the right food to keep going all day – it’s worth looking into that…
After all that – You should definitely do a long walk if you can – the feeling of getting up in the morning and only having a nice walk to think about is close to unbeatable – very good for clearing the mind. I actually found it difficult for a few weeks after I’d finished – I wanted to still be doing it!