I would highly recommend against lack of materials discouraging you. The exercise can easily be done with pencil so long as you commit to your lines. Similarly, you can stroke more lightly or firmly to indicate solid vs intersections.

I agree with @Yasmin Hladky, you are focusing on the wrong thing. You are capturing the contour (or "outline" if you will) of the body - the gesture is totally different. For example, in one image you drew the form of the breasts and another the pecs, but neither of these impact the gesture of the form (I assume - having not seen the images you used). Aim for capturing the 'lines of action', *not* the 'lines of the body'. One thing I believe is having an impact is the size of your pencil strokes - yours are small and tight. Try instead to focus on having long sweeping strokes for a whole section - less is more. If you can capture the whole leg in one stroke, go for it! The two minutes is not to try and be as detailed as possible, but to allow you the time to deliberate more on your strokes before putting them done. No shame in taking the whole 2 minutes for only a couple of lines if they perfectly catch the gesture!

Hi Paolo! @Fat Squirrel below has some excellent points for you to follow. One thing I would focus on for the moment is your usage of cross-contours. It's confusing at first to wrap your head around (but it becomes intuitive eventually - promise!) but you need to think about how any of the cross-sections actually "open" towards you. The best example in your image is in the young giraffe's neck.You change the way the cross-contours curve halfway through the neck - from "opening" down to up. But in the photo, all of those circles should continue to "open" up, because you are still looking down "into" them. Gruesome thought - but if you cut the youngsters head, you'd be able to see into the neck, and so the cross contours should be curving accordingly! More advanced notions will be in perspective, but for now I think really nailing your cross-contours will give you a big step towards the solidity and struture you may be looking for!