You are certainly welcome to make suggestions - or points
JJ already has a HRM, and it is very useful for assessing improved fitness levels over extended periods and also early detection of overtraining / illness. JJ does not 'run to the HRM', it's post-run information only. Also I point I'd make is that 'listening to your body' can result in overtraining because unfortunately some bodies speak in a barely detectable whisper until it's too late.
His training is quite generic at the moment is because he has just done his first marathon and only just selected a set of targets for the coming year. The training programme will now be developed to get the most out of it.
On your next run after the Monday LSR - start with 1.5k warm up, strides, then wait until heart drops to normal, then 1.5k as fast as you can please. Then go for a gentle 5k easy after you've recovered.
JJ, I'm not surprised about the legs being a bit heavy, you will still be on the last phase of marathon recovery and you have maintained quite a lot of training. It should ease up this week / next week.
CotswoldRunner - I had pain / discomfort in lower back and pubic area when doing a lot (for me) of distance work which culminated in a sacral stress fracture. The pelvic / lower back soreness was the warning signs. When it actually fractured the pain switched to a sharp pain when I walked or ran. Hope it's not that in your case but mentioning it because it took months to diagnose, and I had to stop completely for while so ended up taking a year to recover.