I have been primal for about a year now and often go days with very low carb levels (e.g. none to speak of). I have been a "runner" on and off for the last few years, but usually distances under or up to 5 miles.

My approach to running is really methodical and i always attempt to keep my heart rate at 150-155, and I then adjust my speed while running to stay at this level. As a result, I am quite the slow runner

Anyway, i got more serious about this a couple of months ago and am now running 10k distances for long runs, and plan to continue to much longer distances.

I have noticed an objectively quantifiable issue where my running performance, as measured by my ability to keep my heart rate below the 155 threshold and my overall run time performance is significantly worse when I have had no carbs in the last day or so.

For example, a run last week with carbs i did:

3 Miles, 30 minutes, avg HR 153

While yesterday after two days of no carbs I did:

3 miles, 33 minutes, avg HR 158

This is a significantly worse performance and in fact my heart rate spiked quite a bit above 160.

I've seen a lot of folks on here say that carbs are not necessary for distance running, and certainly not for short distances like 3 miles, but I am definitely experiencing this performance phenomenon and have observed/proven it over several months (not just this one off example).

A few questions:

1) Am i doing something wrong? Do i just need to eat a banana before i go running each time?

2) Can i train my body to perform equally well without carbs? Do i need to "fight through it" and expect this to change at some point?

3) Should I just accept the higher heart rate and worse performance?

My approach to running has always been to be maniacal about keeping my heart rate low and below 155 to keep it below the aerobic threshold, as I believe this is the best way to improve distance/performance over time. My issue is that my low carb lifestyle seems to be making that very very difficult to stick to.

Anyone with similar experiences or thoughts on the matter?