Kevin - sorry to slightly disagree with Shades but the Hadd doc starts you off at "75%maxHR or lower".
I agree your real max is probably 5 higher than your parkrun sprint finish. 189 x 75% = 142. In the early weeks this will be incredibly slow and you will lose form, and in one of the other links posted at the start of this thread, Hadd said don't go slower than your 5k pace + 3mm. So say your 5k pace is 6:45, don't bother going any slower than 9:45mm.
You need to get out as often as possible and just jog slowly - once you've had a few weeks of 40 - 50 mpw you'll find your HR drops like a stone. Then start focusing on running sub75% maxHR...and after that throw in some recovery jogs at sub70% max.
NN nice long swim yesterday, and nice double...oops single... run today
Stuart well done on the hilly 12, nice pace. Ice the knee...
Another Serpentine club session tonight, 8.5m with 7m at MP. Actually found two guys who wanted to keep even splits, so very happy with the run. HR still above where it should be for "MP". but I think it's just down to my lack of miles and lack of long runs in particular.