I've reached Level 25 but my mileage bar has stopped moving. Is that normal?

When you reach Level 25 in Zwift, you've reached the last and final level that Zwift currently supports (as of 04/2016).

There will most-likely be additional levels, achievements and jerseys added beyond Level 25, but for now, that's the last level, as the Zwift development team focuses on adding new features and capabilities to the product.

There's nothing wrong, nothing broken, and Zwift is still tracking your additional mileage towards your Mt. Everest, California and Italy Challenges, but it's just not showing up in your heads-up display (HUD) in Zwift. 

Keep on riding! 

(Credit goes to Mendelea Ma for the above screenshot)