Bruce Lee was an amazing martial artist, since his short career. And if you do some of the same things that made him great, you'll be able to vastly improve your own martial arts abilities. Although you may not get as good as Bruce, if you work on these ideas you'll definitely improve.
Develop Great Cardio
Bruce Lee went running every day. Sometimes it was jogging, sometimes sprinting. And he also practiced a ton of martial arts, and did shadow boxing regularly!
So, Bruce was in phenomenal shape. He made sure that he was not just more skilled and more muscular than his competitors, but that he was also in way better shape than them.
Bruce became obsessed with conditioning after a challenge he received where his opponent ran away, and Bruce had to chase the guy down. After several minutes Bruce chashed him down and beat him, but Bruce was ashamed of how out of shape he was, and how much effort it took to win against his unskilled opponent.
To develop the same kind of ability, I'll add running to your regular workout. Better yet, do some cardio exercise that is closely related to your maritime art. If you box, do shadow boxing; If you practice swordsmanship, go through forms quickly or practice specific attacks and parries many times. These exercises will build the condition you need to get better at the skills you do.
If you've ever seen a photo of Bruce Lee, you'll know what I'm talking about. The man was ripped!
Weight training will give you much stronger muscles. Aside from just looking good, stronger muscles will help you pull off your techniques, and also help you fatigue at a slower pace – since each movement is easier.
Now, building muscle by lifting weights does not happen overnight. It takes dedication, but it does work. Start with a well thought out, general program to strengthen your whole body before you start specializing in specific muscles for your art or activity.
Part of Bruce Lee's amazing fighting abilities came from the fact that he was flexible. You can see this if you ever watch the clip of him warm up before his fight with Chuck Norris; He's amazing, and can touch his head to his knees!
Being flexible allows you to take the fight wherever you want. Whether your opponent is shorter than you, taller than you, or you need to get just the right angle – you can compensate, move in, and win. You can also get way more power out of your hips if they are limber than if you're all tight and can barely move.
Like weight training, stretching really takes time. You will not be able to do the splits after just a few days of stretching – it'll probably take you about 2 months if you're young, and longer if you're over 30.
So, work on it a little bit at a time. And if you take this path, you'll slowly, slowly get more flexible. And strong!
Make It Part Of Your Life
One of the cornerstones of a Bruce Lee fitness regime is to make it a part of you life. Working out and getting more flexible is not a sometimes thing, it's an all the time thing!
Do stretching when you watch TV. Be committed to your workouts, both weight training and cardio. Work hard to create a body that will support your activities, rather than a body that drags you down and inhibits you.