I had minimal upper body strength when I started yoga, so I initially did chaturanga with my knees on the floor and my hands in a push-up position, and even then most of the time I could only lower myself rather heavily to the floor, rather than hovering and pushing into Up Dog from there.

It was a long, long time before I managed a semi-decent chaturanga (though I suspect it would have been a lot shorter if my practice had been less intermittent and inconsistent).

Now I'm working on improving my form and getting my elbows stacked right over my wrists, and if I'm tired I'll drop my knees back to the floor to keep the alignment right.

So even after years, it's still a work in progress *g*. On the plus side, regular push-ups now feel easy by comparison ...

On the subject of push-ups, Krista of Stumptuous has a great article on Mistressing the push-up, starting with wall push-ups.