I lost almost all of my stamina in just a few weeks

We need more detail. Have you taken time off? Have you significantly increased volume? Could you be overtraining? Have you been sick? Do you have any flu-like symptoms or other symptoms? Did this happen gradually or all of a sudden? What has changed over this time period versus what you were doing when you were running 13k?

Try calling lost and found to see if anyone found and returned your stamina.

It sounds like you stopped running for some reason.

A running book that I read in the late 1990s, if I recall correctly, reported simultaneous there was now scientific studies that could measure the effects of detraining and that there was not much data.

I read it 25 years ago, so maybe the biophysics people have done more work since, but the main idea I got out of that section of the book is that it is easier to lose fitness than to build it; and that certainly matches my anecdotal experience.

I've been running a bit less recently but it's been a pretty sudden drop off. The week after I did 13k I had to stop after 7k.

A week shouldn't make much difference, even if you stopped running completely.

You have already dismissed the heat, so the other variable could be pace.

How fast are you trying to run 13K? Would you have to stop if you were going slower?

heat can do it.

also, in my limited XC experience, crap diet + mileage can = anemia and other metabolic issues. i got anemia, no high gears, tired all the time, fell off minutes for 5k. rested, ate well including iron foods, strength came right back.

You never had any stamina because you're a useless spammer. You suk at running and everything else. Go away.

You got this,,,

Have your iron tested. What you describe may be a result of anemia.