Yes, those photos in the holiday brochures make it look very good fun. But before you book that 3-week trek, make sure you know what to expect! You don't have to go half-way around the World to find good hiking, you can explore nearer to home first.
A good starting point is what is known as a "pub walk", which is justifiably popular in the UK. You drive to a good pub, walk for an hour or two in a scenic circular route through the surrounding countryside, returning to the pub for well-earned refreshment. After the exercise, the food and beer taste even better! There's a set of short books called "Pub Walks in ..." which detail the good walks and the good pubs for dozens of areas in the UK - check them out at your local bookshop.
Very little equipment is needed for these kind of walks, just a decent pair of footwear, and waterproofs as a backup. It's a good idea to take some water along to keep you going back to the pub.
A step (or two) up from the pub walk is a hike, or a walk - a longer trip, deeper into the countryside and on wilder paths than your average village lane. Still not difficult, but a bit of preparation and some map reading experience would be useful. You can get some good ideas for adventures in your area from dedicated walking magazines, such as "Country Walking", or in the many walking books available.
Get some good walking boots, good windproof waterproofs, a map and supplies. Joining a local walking club may be a good way to tag along with more experienced hikers, or pick short routes to start with and build up your confidence.
Many countries also offer exceptional long-distance walking paths, making it possible to extend the day trip into an overnight or multi-day hike. This could mean camping, or it could mean staying in a pub or a bed-and-breakfast, it depends how heavy you want your pack to be, and how warm you like your nights! The South Downs Way in England, for example, is a superb 100-mile trail from Eastbourne to Winchester which takes you through a constantly-changing landscape of sea cliffs, high ridges, agricultural land and woods. Although the route manages to keep to remote areas, many eating and overnighting possibilities lie within easy reach, and public transport makes getting there easy. And don't forget, you don't have to do the whole thing in one go! It's often easy to piece together one- or two-day stretches, giving a great sense of achievement over several weekends.
Trekking is of course the in-word for any kind of adventurous walking, whether through a jungle or up a volcano. It's also used for multi-week hikes through places like Nepal or India, but even then there are very big differences between treks. A "tea house trek" is one of the more comfortable varieties - walk as far as you feel like and then stop at one of the multitude of cheap, friendly, basic "tea houses" to stay the night. It's even easier if you hire a porter to carry your pack for you, although that's not for everyone. Treks cover the whole range from a few days strolling on the flat, to four strenuous high-altitude weeks.
Winter walking is something of a growing trend, and more and more ski resorts are using machines to clear and pack down special winter walking paths to cater for it. They are often shorter than a Summer hike, because of the terrain limitations, and the generally slower walking speed on snow. However they are still a very enjoyable day out, and provide that "Winter Wonderland" scenery that you don't get from the melee of the ski lift queue.
Several hikes are described here in this hiking section, with a variety of jaw-dropping landscapes and various levels of difficulty. Check out the possibilities in Switzerland, Germany, Madeira, Nepal and also in Britain. Many of the hikes include photos, 3d GPS plots and waypoints.