It's not what you think. You can't use a death plummet to gain distance. No matter how far the new starting place is away from the plummet place, no distance will be added.

So how can death plummets be of any help for the distance race? Simple, some of the plummets put you in exactly the right position to go downhill without having had to go uphill.

Here's the plummet distance race route I'm working on now. I've not beat my top distance yet, but I have got reasonably close; my problem is that I need to master getting close to going to fast without actually going so fast that the bike stops. Anyway, here's the route:

(1) Start at the top of the bridge, and go down on the dirt road or the grass, whichever you like.
(2) Turn off the road and head towards the launcher ramp. Launch yourself.
(3) Once on the other side, follow the dirt road that runs by the mountain until it starts to curve away, then head for the black road.
(4) Go down the black road until you are nearly at the bridge then turn onto the grass section near the sea where one of the balloons is.
(5) Go past the balloon and plummet into the sea. You will find yourself at the top of the bridge.
(6) Repeat until time runs out.

This route contains only a few uphill sections and none of them are very steep. The speed boost from the launcher ramp takes you easily over the uphill section leading to the road.

There are probably many other routes that can be crafted from the other death plummets. Give it a try and see how it works.