You'd probably be wrong on this. RJ's "heart disease" didn't have the more usual kind of progression most heart problems had. His heart was just fine until his amyloidosis started to deposit proteins in his heart, eventually causing sudden cardiac and other symptoms. It was fairly alarming symptoms at that (RJ got momentarily blind on the Memorial Day, it's how it started) and he got himself checked. It took many months before he got a final diagnosis, toward the end of the year, after the KOD tour.


And yeah, RJ had the reputation for being a fairly active outdoor man to compensate for the fact he sat all day when he wrote, for Harriet having convinced him years before to adopt good eating habits, and for having full checks-ups regularly. He had his last one, and got his clean bill of health, a few months before the Memorial day incident (we know, because he blogged about it - there had been rumours he was sick around the time, but it was just that: false rumours). If there was ever one thing RJ was very candid about, it's his health.

RJ's writing pace remained fairly constant. As Adam explained, it's publishing that caught up to him. TDR was the last book he had very advanced before the previous one was released (TGH was completed before EOTW even came out) and by the time of LOC Tor has so squeezed the publishing/longer and longer touring/writing/editing schedule that RJ stopped being able to even outline/plan the next book while Harriet edited the current one, and RJ then have to devote a few "gestative months" to do that before beginning the novel itself (which he did for KOD while writing NS. He got much grief for spending time on that short novel, but he always denied it had any effect as he needed that time to let his ideas for KOD brew a bit before writing it anyway. Those times when a writer don't write and must let his ideas brew as they come is always underestimated... GRRM probably had tons of those while writing ADWD, forced to stop for weeks while his ideas to solve this and that matured...).

After COT (a book that a few months of editing and fine tuning could really have helped) RJ put his foot down and insisted that Tor let him have his full six months cycle of creative editing after he finished writing, which they had cut after LOC to publish his books almost as soon as RJ and Harriet had put them together (Harriet and RJ used to edit the book after the writing was done, but after the mid-series Harriet started editing while RJ still wrote. The effect this had on the series's quality could well be underestimated, ie: that's when structural and pacing problems started appearing, when RJ and Harriet worked on the individual parts, but no longer had those months to step back and look and rework the whole afterward.