I played the original version back at the beginning of the jam and played the updated version a couple times throughout, and the progress is impressive! It's gone from feeling like a classic flash game to feeling like a phone game. The polish adds a lot, and that song is doing some heavy lifting with keeping the game feeling energetic. The endless run is definitely the way to go, absolutely. My current high score is 24869.2.
If you're planning on continuing this later, I'd say all it needs at this point is a bit more visual flair. Maybe add some scenery to generate near the track so it's not just train tracks in an endless sea of green. An updated UI would be good as well, maybe have the speed change size as you speed up and start flashing as a warning when you're going dangerously fast. I could seriously see this as a phone app with a bit more polish.