Hold down the jump button when the treadmill isn't moving in a way you like. From that point, all you have to do is watch the spikes and time it so you land in front/behind them and not on them, and the wind speed. Only try for running on the treadmill when the wind helps balance you out, or if it's not pushing you one way or another too fast.
Does not jumping get you more points?
I always jumped and tried to run as much into the treadmill as I could... I assumed that was the way to get the most points, since I got points for jumping and direction... Well, I got the 50k alright, after a few tries.
The only trouble with that strategy is when the spikeballs speed up, in which case you have to stop for a moment until the spikeball is at a good place to jump over - as long as you don't die in the process.