To be clear, you're not expected to finish it. These are drills. All you're really looking for with the 30 second drawings is to get a good sense of the flow first - draw your line of action. After that, you can work out some of the basic feel of the different muscle groups.
Definitely, definitely study anatomy if you're not already. Loomis's "Figure Drawing for All Its Worth" is a nice little reference, but I find it's easier to go to more streamlined sources. I like Christopher Hart's "Human Anatomy Made Amazingly Easy" and Solarski's "Drawing Basics and Video Game Art". I've also had mixed success with some Udemy courses. What's important is that you're just getting a quick feel for how these muscles and bone structures look - gesture drawing will get you in a good head space, and also teach you about how those structures move.
My sessions look like:
30 seconds: Crude lines of action with circles and oval heads
1 min: Some fleshing out of actual forms
5 min: Crude sketches
10 min: Occasionally more detailed sketches
30 min- 50 min: Almost finished sketches - or at the very least, mapped out structures with more consideration for how light affects the parts of the body (planes).
