This is not complicated Jon. You got tired. Sounds like you are ready for the IM taper soon. I look forward to your smoking me in Lake Placid.

it won't be me smoking you I'm afraid!? I am doing IM Switzerland!

I had similar experience lately. When I recently did a long run at lunch time with temperatures over 90F (found no other time) my heart rate steadily rose (from 125 to 140; 170 is my race HR / LT) as long as I run in the sun. When I came in a forrest it came down again. So this was the heat.
When I did a Half-IM a month ago I felt also a bit light headed and cold. This was after I run the first 9km in the sun at 2pm on a hot day (also above 90F). Then I started to eat some salty food in addition to water and electrolyte energy drink at the aid stations. Then I felt better. Here it was the heat and probably to less salt. But I am not sure.
I also experienced this feeling light headed and cold once I walked for some time trough Bangkok on a hot day.

Felix

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http://www.weilenmann.ch.vu

Classic symptoms of dehydration. Feeling cold is my signal that it is time to stop, no matter what - I take it as past normal dehydration and into a danger point.

When do you do the last long run and bike? I am also doing IM Switzerland and am not sure of this.

Are you Andrew Jonathan (No 219) from GBR. Then we start in the same age group.

Felix Weilenmann (No 169)

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http://www.weilenmann.ch.vu

Yep - that'll be me.

I am no expert..... but.... Did last big cycle w/end last w/end. 110 miles steady Sat, 60 miles of Surrey hills on Sun. This w/end going to Henley Royal Regatta to see friends :-), then Sunday - hills again and final long run - drinking more this time! - somewhere between 2.5 and 3 hours on Monday - then taper from there.

I am leaving the last big run to later for work timetable reasons mainly.