First the speed: well it can have a range, a wide range. Just not too fast and not too slow.
Then you should add something in it to keep you moving, let's call it the Groove.
Then it goes on and on and repeat itself .... No progression can last forever, but it gives you the feeling of eternity.
I always thought that there is something, another concept behind this repetition. A concept that makes it so enjoyable and yes .... It makes you feel eternal ... It makes you eternal but for a short period. If everything goes well that short period will be erased fr ...