The main mode in Super Mario Run is called World Tour. The free version features 3 levels from World 1 and a 20 second trial of the castle level from within the same world.
The good news is that paying $15 unlocks all 6 worlds and 24 levels including 7 castles. Whilst that seems like not many, it’s important to remember that there’s three sets of five coins to collect in each world that are incredibly hard to get and there’s also Toad Rally which allows you speed run against other people on the internet in order to gain Toads.
It’s also important to remember that there’s currently six characters who all control quite differently. For instance, Toad is a lot quicker than Mario and Luigi can float for longer. This in itself makes World Tour replayable.