Warning: Hitchhikers May be Escaped Convicts
OK, the final level. Yes, this is just like a movie, where the final frantic scene has tons of excitement and tons of things blowing up. It's a warthog race to the finish and you have to get through it in 6 minutes. Don't think you have 6 minutes for the original distance she shows you - when you get there, you have to go another 1k or so.
So jump in the overturned warthog and start racing. Don't worry about making all the jumps, half the time it's faster to go the lower route. When she tells you to stop, DO NOT STOP. Just keep driving like a madman. You'll need to do practice runs a few times to figure out the best route. For example, in the first room the best route is over the top - leaping from ledge to ledge right down the middle. In the second room, it's along the left ledge, a straight shot. Then in the narrow tunnel, you take a sharp left partway in for a shortcut to the next room. And so on. The second half is pretty much a straight run.
The person that is supposed to pick you up never shows up, so don't even stop when you are told to. Just drive straight and fast. You'll get told that there's a jump coming up, and then you'll launch into a giant room that you start falling in. This is NOT THE JUMP. There is no way to reach the ledge with the barrels on it. This is the momentum building for the jump. You fall down into a tunnel, land, go forward, and THEN you jump. The jump is really easy to make.
You have to keep racing forward, and eventually you get near the ship and it's blocked. Jump out of the warthog and run like mad those last few steps to get to the ship.
You fly off, Halo breaks up into little pieces, and she only hears 'dust and echos'. But you know it's not over. You know that around the corner is ... Halo 2!!
Note that this is definitely possible on legendary, and if you can do it (just this level, you don't have to do the rest of the game on legendary) you get a special bonus video clip!
Halo 2 Walkthrough
Halo 3 Walkthrough