Being a big guy my self (190cm - 120kg) i suggest you do lots of running, and no weight lifting that will shorten your muscles (you need to stay as agile as possible). You can do sort of aerobic exercises and and work with makiwara for accuracy. Speaking of Karate, of course, a very good aerobic exercise is KATA! Repeating the same kata for 60-70 or even 100 times a day will teach your muscles to respond using the kata moves as easily as you hit a mawashi geri. No matter how strange and complicated your kata moves appear, you ll discover that are truly effective! Repeating kata so many times will also strengthen your mind very much.