Establishing a new "baseline"

Life is never dull and as I get older I certainly take it less for granted!  As a result of my prostate operation, which has been a complete success, they found I have a faulty heart value but are not too bothered about it.  However I have Atrial Fibrillation (A Fib) which is an irregular and sometime very fast (200 bpm) heart rate - fortunately not life threatening.  I started on medication a week ago and it's meant to sort the rhythm out.  
As a result I'm paying very close attention to my heart rate when exercising and trying to keep it down by running slowly and on flat terrain until I can establish a new baseline for my heart rate.  But it does play on your mind?  It sure does; am I out of breath? am I tired?  Should I be running this far?  Not good when the Manchester Marathon is 3 weeks away and I am hoping that things will have settled down by then.  But any chance of a fast time is highly unlikely and probably not worth taking the risk.
But I am beginning to enjoy being back out running again with the lighter mornings and nights.  This is when I feel alive, in spring heading into summer although I've had a couple of colder than expected runs this week.
I'm also back in the gym to build back my strength and try my best to ignore not only the "confinement" of a gym (indoors with lots of people), but the twats who think they own it, hog the machines and free weights while they swan around talking to likeminded twats while not using the equipment!  I appreciate that "gym life" is a big part in their life and better they are doing something positive with their time.  But come on lads, a bit of consideration to the other gym users please.
Consideration is also in short supply as you try and dodge the long dog leads or the dogs who are not on leads at Strathclyde Park as they get in your way when you are running.  It's not the dogs fault and I love dogs but come on owners, runners really can hurt themselves either trying to change direction quickly or even worse falling over your beloved dog which would be a disaster, especially for the dog.
I think I've got all my moans out of my system.  I put it down to discovering that getting older brings with it some challenges.  But I love a challenge and nothing's going to stop me.  Not even a long lead with a handbag dog on the end of it.  Bring on Manchester.  After all it's meant to be a warm up race!