Fatigue or poor data?

Today's run explored a path between Burbage edge and Oxstones. I've seen people arrive at Oxstones from the east so assumed there was access. I've previously looked for the path but not had map with me, today I had my cheap phone with Viewranger with me so could relate my position to the map readily. The Strava heat map shows this new path nicely.

The path across the open moorland is indicated

With hindsight, today wasn't the best day to look for this route as there had been a lot of rain and so the path, and the moor was waterlogged. It was impossible to avoid wet feet and muddy legs.

The west peat, mud and stony ground forced me down to a walk as I was fearful of hurting a knee or ankle, pretty much my usual excuse to go slow. This slow but steady approach yielded the typical pace distribution on Strava.

As I do consider I'm still recovering from 'The Big Walk' and I had a good swim yesterday that was much as expected. The odd thing is that my heart rate data is to put it mildly, ridiculous.

Now I'm try got decided whether the data is rubbish, that is the wrist mounted heart rate monitor didn't take valid readings, or whether I was working that hard. I understand the 'threshold' section as on the road ascent I knew I was working hard-ish, but the majority of 'anaerobic' feels like rubbish. Clearly one can't be anaerobic for an hour!

As I type this my heart rate is 53 which is a little higher than I'd expect so maybe the data is indicative of underlying physiological stress?