My fitness seems to fall off a cliff if I stop training for a relatively short period of time, especially with my running. At the end of my triathlon season in mid August I took a couple weeks off and when I started up again my run speed/HR was the slowest/highest it had been all year (or maybe more) on my easy runs, and it took awhile to get back to somewhat close to where I was before so it wasn’t like a one time HR oddity (I’ve used HR for years and have a pretty good idea what’s what with my body and very rarely have issues that HR naysayers coughTRcough 
like to throw about). I took 5 days off over Christmas and on the same treadmill run I did 9 days earlier (and many times prior to that or runs at the same speed) was suddenly 7 beats higher on average and my follow-up runs have also been lackluster. My bike fitness doesn’t seem to fall off as fast though, thankfully! Anyone else experience similar issues?
Some info on residual fitness.
Many years ago I was told from a Sports science graduate that you roughly lose fitness six times faster than you gain it. Not as in depth as previous answer, probably no were as near as accurate, but easier to remember and works for me…
I have been following this website and the findings and discussions of the author. This intro has an interesting case study regarding post season fitness and training that highlights how quickly fitness flatlines. I’m finding all his posts informative and worth reading.
Interesting stuff! I definitely don’t do enough high intensity work on the run, especially during the offseason, since I have a high risk of ending up injured - - I’m much more likely to throw those efforts in the bike since injury risk is minimal so that could be one of the reasons why my cycling doesn’t fall off as quickly.