why was my easy run so much slower today

getting a fake vibe here (80% chance this is fake), but since I'm trying to avoid work for a few minutes:

if you have a race this weekend, then take off completely until then because you will run like garbage if you're sore and injured, and there's 0 fitness benefits to be gained between now and this weekend.

if your form is being affected by an injury then you should either be doing work on that injury so that you're able to run without modification, or you need to take a break from running to heal enough to run unaffected

BECAUSE YOU'RE A WIMP!!!!!

That's what I said, yep, did it. How bout that?

If I run late-ish, finish with stuff that floods my legs with lactate (i.e. 200s at 800), and don’t cool down to pump the lactate out, my sleeping HR will be 5-10 beats higher than normal

I often run 9 pace on regular runs and am in 17:20 shape.

One day does not make a runner. Off days are totally normal. It could be for any number (or combination) of reasons. Please don't overthink it.

Take ANOTHER easy day with a couple of strides at the end.

You will be fine for Saturday.

^^^ The above advice is based on 50 years of running and racing. If you didn't ever have off days, THAT would be a be more of a surprise.

heart rate varies a lot day to day,
you should be training by effort rather than an inconsistent number

Because you ran easy and easy is a feel not a pace. Don’t fret you’re doing good. The moment you start to worry about easy pace is when training starts to go south. Totally normal just focus on workout days

Source: occasionally run 9+ min miles as a marathon OTQer

What are your race times? There are lots of reasons that we sometimes struggle with paces that are usually easy for us, but I can give you better advice with a bit more info on your current fitness.

Either you'll wake up sick tomorrow with a cough/cold/flu or you just need some rest. Rest a bit and see where it goes.

Put the watch down and run by feel. I turn off my HR on my watch and run by feel. Reading too much into metrics can stress you even more. Learning to listen to your body is very important

if i had run by feel, i would’ve gone the same pace. i felt terrible, like i could barely move and my breathing was heavy.

3k pr 10:33, xc 5k 19:25

thanks for the (helpful) responses!

i notice this too often. I would be careful about overtraining---if that happens again i would take a few days off

There is absolutely no way that your fitness decreased significantly in a matter of a couple days when you are still in training.

Keep in mind that your fitness level is just one (albeit very important) component of determining the pace and effort combo on a given day, but your fitness tends to evolve slowly, not make drastic jumps, especially backwards, while in training.

This means that a drastically slower pace at a slightly higher effort level is likely the result of a combination of these factors:

-Illness - you could be about to get sick, or your body could be expending a tremendous amount of energy behind the scenes to fight something off.

-Overtraining/Fatigue - You may not have had one big, killer session, but maybe you've been putting in solid work for awhile, and your body needs a chance to absorb it.

-Issues specific to females - I'm guessing your are either a HS or college athlete, and as a female (especially in that age range), you might encounter times when your body is preoccupied with things that are not conducive to athletic performance.

In any case, your body seems to be telling you it needs some rest (which can still be active, just reduce the load and intensity). Listen to it. If you aren't feeling better by the day before the race, consider not doing it - it's just one race, your long-term mental and physical health is more important.