Use the pause button. Dear god use it. I found that one helpful tip is that after a little hard mashing, pause the game and get your stamina back and then carry on. I had to pause 4 times during the race so i could rest my thumb.
As for button mashing, i use my thumb. What i do is press the controller against my right palm so the control doesn't move about, cramp up my right thumb (it's uncomfortable but works) and then just jab the X as fast as possible.
I was knackered at the end of it but pausing at various intervals helped. Also i found standing up more helpful although i think i ended up standing up anyway due to concentration i was at...