Once I knew the length of my final project, I opened up Adobe Express, clicked the + sign, and chose video as my project. Then I created a slide with a title graphic, set it to 30 seconds (the maximum duration for a slide in Express Video) and did a bit of math to determine how many 30-second slides I would need to equal the length of my audio project. The I duplicated my first slide 19 more times for my first segment.
That duplication process is a bit tedious as you can only duplicate one slide at a time, but it's easy enough.
Once I had all my slides, I chose the Music option, and chose to Upload My Music. But, instead of choosing a song... I chose my narration file. I know ... sooo sneaky!
Note: You may find you have to tweak the duration of the last slide. It's important that the slide doesn't run longer than your audio project, or (because it's being treated like music) your audio will start to repeat.