As per the other comments I think you are on the right lines.
I don't think yo uwill properly recover from your Thursday session for a proper PB attempt on the saturday. If I were you I would aim to go for a PB once every 4-6 weeks and taper for it. Your current thurs/parkrun plans are fine for training and improving
Also after hard intervals I would tend to have the next day as a rest day or x-train and do say a long bike ride wo swim.
I would look at training load as a way to determine your PB readiness (this needs 45 days HR data and your HR Zones)
I would also agree with @ross comments on determing correct paces. Although with longer runs HR can also be useful.
I would also urge you to vary your trainign week on week. Perhaps whilst sticking to the general concepts you have developed.
don't take this the wrong way but you are no spring chicken. So you need to remember to include periodivcity. Your recovery within your week is proabbly OK in reality perhaps the week after your 4-6 week PB attempt I would have it quite a bit easier for the whole week before ramping up again.
why not use one in the local gym and then go flat out for 20 minutes. divide your average wattage by your weight in kg and that will give you a meaningful figure to compare with other people. Look for CP20 and watts per kg.
i did it a couple of years back. nice area. flat run and undulating bike. still the std distance is 2 hours + for most people...more than a walk in the park. also i think it is this years national champs so the AG Kings will be in there as well....a good event to see how you compare.