Originally Posted by AWP
I'd only recommend that you have a 'goal' for each repeat, within each repeat or blocks of repeats.
I'm with Alan on this. It's a common, and I feel short-sighted (or more accurately short on imagination) practice among marathoners -- and some coaches of swimmers more youthful than us -- to do sets that seem designed mainly to test what's the maximum number of maximal-length repeats you can do. Sets of 15 x 1000 or 10 x 1500 are actually not that uncommon.
But it also becomes a test of one's ability to maintain your sanity. Most people do so by either tuning out entirely, going on autopilot, or distracting themselves with mental games that have little to do with the task at hand. It's the antithesis of mindfulness. Also of Deliberate Practice.
I abhor the very idea of any training that encourages autopilot, and have never done anything remotely like it while preparing for my two 47k Manhattan Island Marathons or my 40k Tampa Bay Marathon. In fact, I thought it much more critical to train for mental endurance (the ability to maintain a targeted focus on--or stay present with--each stroke for a swim of 8 to 12 hours, than to train for metabolic endurance.
A possible task for such a set. Can you swim an SPL pyramid within a 1500?
EG. 40-41-42-41-40 SPL by 300s.
Tempo Trainer and SPL could also help with focus.
And as John suggested, it's a good idea to start with just 3 to 4, especially if you've never done 1500 repeats. it will be easier to create tasks that require full focus when you do 3 to 4. And you'll have a good idea of how good your mental endurance (ability to stay on task) and motor endurance (ability to resist efficiency degradation) is on a more moderate set. My guess is that you'll find that a set of 5000 to 6000m gives you all you need.