CSI Mark - I have very similar symptoms to you - including the low resting heartrate (but not, as far as I'm aware, the 3 second pause). I had ECG, echo and 24 hour monitoring (which caught 373 PVCs).
I saw a cardiologist, and he said (a) these sorts of symtoms are only very rarely indicative of a problem; (b) no problem keeping running; (c) that he is sometimes called by surgeons worried because they're wanting to operate on somebody with an irregular heartbeat - he tells the surgeons to stick the patient on a treadmill (assuming they're well enough), precisely because the irregularity tends to disappear at higher heart rates; and (d) that he spent some time working as the cardiologist for the Briish Olympic team, and that the athletes' ECGs were a "bloody nightmare" - and that many of them barely had a normal sinus rhythm (because their hearts were so well trained, high vagal tone, low heartrate, etc).
I once ran the Mitcham 25km (reasonably quickly, as well - 5:45 miling) with my heart flip-flopping all over the place for the first half of the race (as I became more tired, and my heart started to work harder, it became regular), and although it was a little disconcerting, nothing bad happened. I sort of decided then that if I was able to survive that kind of thing then probably I was going to be okay.
I had a whole load of tests last year because I was getting PVCs (premature ventricular contractions) or "skipped beats".
24 hour holter monitor caught 350 of the buggers. Had an echo, ECG, etc.
Saw a cardiologist. Basically said that if you have a healthy heart, then PVCs aren't a problem. Moreover, he recommended running as a way of settling the heart if you're in the middle of a run of the things.
Also, he reckoned that these sort of odd rhythms are more common in people who are highly trained. In fact, he said he once did some basic medical stuff for the British Olympic team, and that their ECGs were, and I quote, "an absolute nightmare".
I would *never* stretch an injured/painful calf muscle. And actually there's no evidence that stretching before you exercise - and scant evidence that stretching after exercise - makes any difference to injury rates.
Calf muscles can be a nightmare. And probably there is no sensible advice other than that you might get away with it or might not.
But, FWIW, I actually hurt my calf muscle in the first mile of my last marathon (i.e., to the point where I was limping with it, and I thought there was no way I was going to get through the race). But it got better as I went along (or possibly it was that other things started to hurt a lot more, hard to tell, but I was able to complete the race).