work up to

(phrasal verb) slowly proceed or progress to something more difficult.

Example Sentences:

If you want to lift 250 pounds, you should start at 100 and work up to it slowly.

Student: I would like to take the Advanced Grammar class.
Teacher: I think it might be a bit hard for you. Normally, you start with Beginner Grammar, then Intermediate. Only after you’ve worked up to it can you take Advanced Grammar.
Student: Oh.

Old Friend 1: What are you doing these days?
Old Friend 2: I decided to go to Pilot School, and I’m working my way up to becoming an airplane pilot.
Old Friend 1: That’s great!