This mode was released on the Dec-15-2025 patch.
Each challenge you complete gives you a pick of a few bonus options to carry into future outbreaks played. Each challenge sticks around for 2 days.
Every time you beat all of the options initially for a day, you will get a bonus challenge. This means you can do 2 per day. If you skip a day, come back, and complete both the last day and today's, you will only get that day's bonus, meaning 3 over 2 days instead of 4.
Now here is the express mode -- You can change your system clock to pull up another day's challenges. If you want to speed through this, complete a day and it's bonus, and then exit the game, change your system clock, and start the game again. Voila, fresh challenges.
So this could theoretically be done in a few days. It is still 100 successful rounds that need to be completed, with a minimum of 50 "days" worth.