As the act of walking over long distances requires considerable physical stamina, you need to prepare in advance and get your body in shape.
Before taking the plunge into the woods, do some basic research into the kind of place you want to go. If you are a novice, you might want to go for shorter, easier treks on milder slopes.
Before hitting the trail, you might want to consult the internet or the local forest authorities about the kind of weather expected on your big day. You can then modify your plans or pack accordingly.
Since you are planning on spending a few days in the wilderness, you need to pack accordingly. First aid kits, fire-making supplies, food, appropriate clothing, tent and torch are few that come to mind.
Last but not the least, don't leave your supplies abandoned in the woods and make sure you leave the forests as beautiful as you found them.