Anyone have any experience with a steep increase in HR nd extreme fatigue after 4-6 miles of running? I started have this issue last year while building for IMLP. After 4-6 miles my HR would go from a mid Z1 to Z2 and then Z3 if I kept the same pace. From a perceived effort, it's hard to even maintain a Z2 level effort consistently without stopping. Pace wise, I have to run extermely slow and even then I have to stop as my legs feel like bricks. I end up sweating really heavy or at least for me I do. Afterwards, I'm a wreck, worse than if I had done a race.

It's been happening off and on since April 2013. Last year I chalked it up to stress as I had a lot going on outside of training for IMLP and I still had a decent performance once i got to the IM. That being said, I don't think I had a good run leading into the race. This was the first time I had this issue.

I'm not going out to fast, that I know. I thought maybe I was going into the run dehydrated but yesterday I had Water - 6-8 X 16.9 ounces of water and one NUUN tablet.
3 X 8 ounces of coffee. On the 1:15, medium intensity bike - 2 X 24 ounces water, one with NUUN tablet. I also had 2.5 glasses of red wine with dinner.

So what gives? I'm also wondering if I'm not recovering from the previous runs and I'm caught in a downward spiral. I'm running 3 times a week, around 20-25 miles and a total of 10.5 hours of training a week, so nothing to extreme.

It's happened every run for the last 2-3 weeks. Also, I live in Florida where it's pretty hot. 41 year old male.