Building up the stamina

I need to do the intermediate part more often. As it is now, I am completely out of breath and my legs are jelly at the salabhasana B!

At the present, I am practicing four to five times a week, two of these with the intermediate, two to three times I do only primary, Saturdays nothing, and the occasional no-practice morning, just the sun salutations. That seems like a rhythm that I can live with. Clearly, that might well build up the stamina for a longer practice, but very slowly. If I want to be less exhausted in the intermediate, I should do that part more often.

That's one thing I have learnt by doing ashtanga: just by repeating a difficult task (asana) X times, by force of nature, it becomes easier. The more you repeat it, the faster that happens. There is no talent nor tricks involved.