20.26 firmware sudden race predictor and endurance score spike

I have the same issue. Similarly, I didn't put much stock in the numbers themselves, but enjoyed seeing the steady progress. Sudden changes undercut the feeling of accomplishment. 

It is intentional, read the release notes:

“Improvements to race time predictions based on fitness level and recent efforts.“

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/383109/forerunner-955-software-version-20-26---20-phased-rollout

For me, mostly the 5K value chanced noticeable, the others not so much.

I know it’s intentional. I was reporting it because it’s now wildly inaccurate vs only a little inaccurate before. 

I see one resolved report of it returning to 'normal' a few days after spiking the other way (worse times) on the 255 forum. Here's hoping it does the same here in a few days' time.

Same here, the predictions for all the distances suddenly jumped to extremely optimistic values

Before this release race predictions were optimistic.

Now they are simply fool!

My PB are 18:21 for 5k, 38:24 for 10k and 1:25:36 on HM done 1 year ago at 47yo

Same here - I set a 5k PB a few weeks ago of ~18 mins, when my watch predicted high 17s. Suddenly I’m apparently capable of 15:xx which is just ridiculous. Really frustrating regression from this update that renders that feature completely useless 

I raced a 5k in 17:30 last week. Garmin predicted mid 18's. Now Garmin predicts 17:01. Same vo2max of 58. 

Lol. I was just going to create a post stating the exact opposite reaction to the same problem.

The update came in as I synced after my morning run.   On my second run today I noticed an improvement from 2:56 to 2:48 in the marathon which is a huge jump in one reading, especially  considering I've run 2:38.  

I've never ever put much stock in this, and in fact I've had my fastest race predictions only when I took 3 months off for injury in the last year (why does running zero for 3 months improve fitness? was I THAT over-trained? )

- seriously happy runner 

For real though I really don't put much stock in these numbers.  It's a meme between my friends as who can beat their marathon predictor the most.  On average we're 20-30 minutes faster than the old predictor.  

Myself and many of my friends are dramatically faster than Garmin, so this is much better for us.   We often joked say we ran a 34:xx 10km and garmin would say'...err. 37? 38?' we'd say, 'what, like right now? post run?' 

lol. Yeah, I definitely don’t put value in it either. Hilarious that it was underestimating your marathon times so badly before when lots of us probably had the opposite. You’re not doing a 3 month taper? You’re doing it wrong. You’ll probably break the world record once you correct that hahaha. Clearly not a great algorithm.

Tbh I only checked it to see what it was estimating approaching the last marathon I did, as sort of a comparison to the same point in this training block. Maybe it’s just using VO2 and not factoring in that I’m probably an inefficient runner at my VO2 max  

Yeah, seriously annoyed maybe over the top. Definitely annoyed by the OHR mess though. That’s a joke and they should be fixing that