@Indy -

All intervals - all training - causes muscle damage. That's the point, to create microscopic damage so muscles and bones re-build themselves stronger. But you only want to damage them to the extent that they can quickly recover. One thing I've always heard is that if you're sore the second day, or if soreness limits your next workout, you did too much - but by then it's too late.

I'm fuzzy on the theory of why long intervals are so hard on your body, whether it's the old-fashioned idea that staying too far above your lactate threshold for too long, without an interval to clear it, creates a damaging lactic acid buildup, or something else - but I know for sure that of all the workouts I do, the one that makes me the most sore is 3 minutes effort/3 minutes rest. By the time I get to mile repeats, yeah they're hard as !@#*, but the intensity is lowered enough that I don't risk feeling sore for more than a day.

If you're upping your cadence, you might invest in a clip-on metronome, or if your HRM has a footpod, set it to display cadence, or even [shudder] put an earbud in one ear with some 85-90 or 170-180 bpm music....

I'm a total evangelist for Chi Running, but I know you're a yogini, and Chi Running really has a lot in common with yoga. (You probably know it's derived from T'ai Chi principles, another meditative practice.) It's true there's a lot to "think" about, especially when you first learn the method, but it's body thinking, aka feeling or sensing. I'm the kind of person who learns physical skills by doing much more easily than by reading or watching, and if you're the same way, if you're interested, I'd really recommend doing a workshop if you can. I don't think the book would've made a bit of sense to me if I hadn't done the workshop first.