Just do the activity often that you want to build endurance for, be it running, zorbing, copulating, or whatever :-)

well there's several types of endurance, mental, cardiovascular, muscle... Mental endurance means you're required to focus and concentrate for long periods of time. I'd advise you use memory games, focus games, like the ones you can get on hand held consoles. Play them several times a day. Play complex card games, learn new games, speak in an accent for as long as possible... build up to it though. Cardiovascular endurance is more commonly known as fitness. This would be using rowing machines, running, doing a varied workout, but long periods of time. Make sure yuou are properly warmed up etc before you start. If you dive straight in to trying to run half marathons, you'll almost definitely cause yourself an injury. Muscular endurance is also known as muscle memory. This is built by basically doing the same action over and over again, on light weights... basically, high reps, low pressure. This is generally done for sports such as rowing, where the same action is repeated several hundred times in mere minutes. My crew trains at chest presses, doing as many as possible in a minute. The aim is to make your muscles have the ability to repeat the same action without flagging, and to keep consistency.