The only real way to plan a long-distance walk properly is to learn from experience, as everyone has their own favourite ways of tackling the trail. However it's a good idea to have a checklist to go through before you start, so here are a few questions you should ask yourself before planning each walk. This list doesn't cover everything; it's just a start.
What will the weather be like? Bad weather can completely ruin a long-distance walk – not to mention making it more dangerous – so make sure you know what the local weather conditions should be for the time of year, and prepare yourself accordingly.
What distances do I need to walk, and over what terrain? If you get the measurements wrong and end up setting yourself a trek that's far too long, you'll risk damaging your body and getting stranded. Don't just take distances from a book and assume you'll be OK; make sure you know what sort of terrain you're going to be crossing, and break the walk down into smaller stages if there's any doubt as to your abilities.
What shall I do in an emergency? Hopefully you'll never need to use it, but it's essential to have a plan in case of emergency. Do you have a mobile phone that works on the trail? If so, do you have the relevant number for the emergency services? It's also important to leave your itinerary with someone you know, or the local park authorities, so they can track you down if you don't check in on your return. Other things to take are an emergency bag, a whistle and a basic medical kit.
What maps and guidebooks do I need? Maps are essential (as is a compass), and I always try to get the most detailed map available. In the UK the Ordnance Survey Explorer maps are second to none, and for most National Parks there are maps available locally. Sometimes it's impossible to get a map, especially in the developing world, but you should at least try to get a feel for the trail from locals before setting off.
What about money? It's always advisable to carry some cash with you, just in case of emergencies. In the wilderness you're unlikely to need money, but if something goes wrong, cash is extremely useful.
There's plenty more that needs to be done to plan a successful long-distance walk, but the best way to become a good walker is to go out there and walk. You'll soon know whether you've picked a trail that's too hard or a pack that's too heavy, and there's no better way to learn about what to do than by actually doing it.