Know where you're going
Secondly: Buy the guide book.
As well as pointing out areas of historical and cultural interest it will explain, in detail, each day's walk. Information that is sometimes invaluable when the waymarkers suddenly seem few and far between. At journey's end your guide book will become a favourite coffee-table book.
Pack quality not quantity.
Every time my partner and myself plan a walk I try to pack less. I have not yet missed an item left behind. The most important thing to remember is your little plastic rectangle with a CVC number on the back. You can always buy an item you feel you neglected to bring. With toiletries, a change of walking clothes and a "make yourself presentable" outfit, I find 10kg is a realistic limit.
You want to be comfortable. You will be walking between 15km and 25km a day, depending on the walks you decide to do, so buy the best hiking boots you can afford, the best raincoat, the most comfortable backpack, wickable shirts, thick cotton socks, etc. These purchases can be expensive but are worth the return in travelling comfort.
Book your accommodation and be sure to plan break days
Walking holidays aren't a race. If you know where you are staying, you can enjoy your day without having to stress about accommodation. The guide books tell you where each day's walk terminates so it is easy to see where to book. Don't be afraid to give yourself a few rest days in towns or villages you think look interesting.
Be prepared
Get fit for your journey. The fitter you are, the more you will get from a walking holiday. Some people train for months. Some people train for weeks. Some people train for days. I admit, I am of the latter preoccupation and often suffer at the beginning a walk. I have found stretching helps. Particularly stretches that loosen hamstrings and calves.
Choose a walk that suits your ability
For a first walk I recommend The Thames Path. It is a two-week walk that begins in Cirencester in the Cotswolds and follows the course of the river Thames to London. As the river widens each day, it passes through historic towns such as Oxford and Windsor (which make brilliant rest days.) The daily distances aren't arduous and the scenery is spectacular, particularly in the Autumn or Spring.
You may be wondering: "What's the point? It sounds like hard work." It can be. But I promise you, no drink will taste more refreshing. No meal more satisfying than the one you sit down to at the end of a walking day. Best of all, you will find yourself reminiscing about places you would never have thought to visit.