You may be at the best shape but if you have no interest in walking, trekking is not for you. With a little training exercise and enthusiasm, you can participate on any trekking trips in Nepal Himalaya. There is no one size fits all kind of training program. Everyone is different. There is no better training than trekking itself. Get a day pack on and get out there as often as you can. But if, like most people are, you are limited with free time, you can do short power exercises like jogging, swimming, cycling, aerobics etc which you need to start 2-3 months prior to the trip date. The exercises strengthen your muscles and builds stamina and confidence for the trekking. If you can manage a couple of valley floor to ridge line ascents per bushwalk, you will cope with a moderate graded trek. Speed is not the essence; stamina, confidence and continuity are.
However, if you are planning to join a commercial trekking or mountaineering expedition you will need to develop a high level of fitness so that you can comfortably and safely endure several weeks on the trail.