Agreed that straight pool is a very focused way to practice. I sometimes keep a score, starting with an ideal break shot and running until I miss, then starting over with another break shot, but vary your opening break shots so you are not practicing the same break shot over and over. Do a 10 inning session, then tally the total balls made for the 10 innings.
It's also very revealing to record the reason why each individual run ended - missed break shot, failing to set up a break ball at the end of a rack, leaving a hard shot or no shot following the break shot, missed positioning leaving a hard shot then missing, missing an easier shot due to taking it for granted, leaving the cue ball stuck in the pack with no shot, scratch, etc. By doing this, over a number of sessions, you can analyze the most common reasons your runs are ending and work on improving those weaknesses.