I am trying to refine MIles Black's lessons into learning actual piano pieces but I am finding jazz harder to play than it looks. For example I am trying to develop an improvised piano version of confirmation and while I can pin out the melody line the chord accompaniment seems more elusive. Also I lack the stamina for some parts. How do pianists increase their stamina? Any ideas?

I find the best way is to take a difficult section, even if it's only a couple of bars, and just practice, practice, practice.

You increase your stamina, and skills at the same time.

Also, take good care with the fingering you are using. Play the section slowly, ensuring that you are using the best possible fingering to get you to the next notes. Poor/incorrect fingering can create fatigue.