One question on that post.. she notes every other day, not every day.
Why is that so vital?
I'm not really in a position to answer. For me, her system is one that allows me to run, and to more-or-less enjoy running, or at least, not dis-enjoy running, and thereby to get noticeably more exercise than I was getting prior to starting what she recommends. I don't know that it is the "best" system and it's clearly not the only such one (the other one that I know of being couch-to-5k), but it's the one that's worked best for me. Her take, and I have been trying to deploy this, is that it's better to do something else on the "off" days. Overall for now, as I've gone from running 0 days per week to running ~3.5 days per week, my take is that this is a good system for me.
At some point it's gotta be fine to go once/day, right?
Clearly lots of folks do. I'm not really interested in doing so though I will say I see the every-other-day thing as a "guideline" not a "rule." Thus far, and I'm all of about a month into this, so there aren't many data to examine, I haven't run two days in a row, but I may this week as for scheduling reasons it may work out better for me to either double-up (across two adjacent days) or skip a day (i.e. space it out to 3), and I figure there's no harm in trying the adjacent day thing and seeing how I react to it.
Overall, my sense is that she's trying to present a good way to use running as a steady and valuable way to exercise (more or less across the entire life span and regardless of how unfit you are) while avoiding injury, annoyance, etc., and that the every-other-day plan is a good one for that context. Which is pretty much how I want to use it, so I'm not tempted to push it too far, so far.