Tails won't find Green Hill Zone familiar (after all, his debut game was the one after Sonic 1. Once you're back in the white void, Tails will be freed from his statue-state. Defeating enemies and collecting rings will slowly refill your meter, so if you feel like you might run out, look around for a quick pickup.
Boost whenever you can to crash through enemies and gain tremendous momentum, but keep an eye on your boost meter. With this different play style, you'll spend a good chunk of the level facing Sonic's back instead of his side. Once you're ready, head to the white structure in the middle of the small amount of space you can move in to play the next level. You can test out two of Modern Sonic's moves now: jump after being in midair to do a homing attack, introduced in Sonic Adventure, and hold to do a boost, from Sonic Unleashed. This is the hub world of the game, where you'll be able to access every Act, Challenge Act, and Boss Battle in the game. The Time Eater knocks Sonic back before he can get close, and wakes up in a white void after passing out. It isn't too long before everyone at the party is also sucked up by this villain. After this, Modern Sonic is treated to a birthday party, where Tails presents him with a chili dog, his favorite food. After getting through the first Act, a pitch-black monster will arrive out of a wormhole in Classic Sonic's world.