Running and Heart Rate

Apologies for lack of updates - I really haven't had a lot to say recently...

Went for a run yesterday, approx 5km and was really struggling.
Not quite sure what the deal is - I run by myself or with the training group and I'm fine - comfortable and feel like I can run probably 8km.

I run with my work training buddy and I struggle through the 5km.... it's either pace or focus, not sure what - but I know I need to work on it.

As for the heart rate I'm a little concerned right now - my theoretical maximum heart rate is between 185 - 191 (dependent on the formula you use to calculate)... in both of my last two runs I have averaged 145bpm and peak of 182 / 177 respectively.

That said I haven't felt that bad running at that level - have felt pretty reasonable so I'm going on the theory that the maximum calculations are wrong and it's nothing to worry about.

As a side note a few months back I worked out in 40 degree heat, in a tin shed, doing a high impact weights superset, at the end of that my head was spinning, I was dizzy and seeing spots... I measured my heart rate there and was nearing 200bpm.

Still am going to see a doctor and just have a checkup as I don't want to be pushing myself so hard my heart is going to pop or anything like that.