Well, just continue regular long note exercises, but with the circular breathing and focus your mind and ears on sustaining that note without wavering.
I think the most effective use of CB is for extended phrases, so practising playing phrases while breathing in is very good.
Circular breathing in order to play one very long note is a kind of novelty effect (often good for some applause, I've done it several times myself). I mus admit that when I've done this as a novelty, I do a big bend on the in breath. I'm not as proficient at sustaining the note as I could be so I do the reverse and don't make it appear I'm trying to keep the note even, soi I do the big bend.
However if you need to sustain the note because that is written or directed, then I'm afraid you must just persevere at holding the note.
Well done for getting over the first big hurdle of actually breathing in.