A few tips which should make it a bit easier:
There is only one achievement that requires you to race on hard difficulty, so always have it on the easiest difficulty.
Make a note (whether mental or not) of what tracks & vehicles you have used, as this will save you from repeating too many races.
You don't have to hold the phone up for the controls to work. You can put it down on a table and just lift up each side. I found holding the phone up all the time made my arms hurt after a while.
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