Been playing this for most of the day, and have finally managed to get to the top of the scoreboards on both Score Attack and Marathon modes. That means Challenge and Time Attack modes are open for me. And are clearly impossible.
I realised today what the point of the dropping-to-the-ground first section is – you build up points, and then the more points you get for that section, the better the Yoshi you land on. Different coloured Yoshis have different running speeds and different maximum egg limits. This seems to be very important for the two new modes I’ve unlocked today.