Is it possible to complete all of the challenges by just normally playing the game without "grinding"?
During the game, I stayed mindful of many of the other challenges, deliberately focusing on a each weapon until completing the related challenge, but in many cases I was only fighting to get my stats up.
I made the unfortunate mistake of sprinting, jumping and gliding around everywhere, so at the end of the game I'm about 2 million miles short on the "distance travelled in alien vehicle" challenge.
I've given up on completing all challenges in this game - on my next playthrough, I'm going to try using an Xor as much as possible - in lieu of any other form of transport.
It just seems like that vehicle distance requirement is excessively high.