Having a big stack in a poker tournament is not the solution to your problems, despite the confidence that it (quite naturally) fills you with. When you get a larger stack, your playing style needs to change accordingly and move from being a tight aggressive player to working with more hands and loosening up your game. Then, later in the tournament, your stack is the single best asset you have.
Early On: Looser Calls, Play Aggressive
To keep your lead in a no limit tournament (or, indeed, any tournament,) you have to keep putting more chips in your bankroll. If you've established a lead early on in the tournament (and indeed, as you are playing in the middle stages), don't be afraid to start making make looser calls from the flop, limping in hoping to hit a nut hand. Later in the tournament, you may leave hands like 67 or 78 to the side, but if you can afford to gamble a bit with your chips and be more aggressive, don't be timid.