Yesterday I did a 75 mile training ride. The goal of the ride was just to keep the HR in my aerobic building zone (for me that's 140-160.) Temps started around 80 and went up to the upper 80s, light wind.
Fuel: Gatorade sips every 5 min, on the 10 min mark, I also had some sips of water. At 25 miles, I had some granola bars.
Everything was going great for the first 45 miles or so, then my HR spiked and would not drop below 165 no matter what I did. I slowed way down, took on more water (slowly) then took a good long rest at 50 miles. After a brief walk and a little while sitting in the shade my HR finally dropped below 130. I took on more water and gatorade and had some more granola bars. I started for the last 25 miles with a HR of 125, the lowest it would go. I have never gone so slow in my life, but my HR averaged 161 for the last 25 miles. I'm not sure what happened.
I'm guessing a combination of general fitness (this has been my longest ride to date, but 60 miles was not a problem last week); perhaps dehydration and maybe something else. I was well rested prior to starting the ride and well hydrated.
I'd like to not do this again, so if based on this I can get any advice on what to change, please help.