With all these matters, the issue which invites program structure experimentation is the need to improve front-end race rhythm without damaging (and indeed continuing to enhance) back-end speed. Individual sessions are obviously important but it’s how they are sited within the plan(s) that enables the athlete/swimmer to take what s/he wants from each session - be it pure speed, pure power, speed endurance, power endurance etc. Again, contrast and recovery within the plan are key and so the concurrent structure enables that, imo, better than the traditional long to short (pyramid) or even short to long (inverted pyramid).