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Having only used ithlete for a week, it's early days, but it sounds like your usual reading is about 80. That's a good high reading & indicates you have a high level of aerobic fitness.
A drop of 18 is quite large, and could have been caused by the ride, though you don't say how intensive the 18m was. It could also be that you did not sleep that well the night after the ride (eg if the back injury is still bothering you a little), or there was some other source of stress that added to the drop.
The good news is that you took notice of the drop & recovered within 1 day of rest.
Over the next few weeks you will get a better feel for what kind of drops / gains are to be expected in your particular case. We're all different - for instance a drop of 5 or 6 is quite significant for me, but my normal level is also lower at about 65. I hope this helps & thanks very much for sharing the feedback.

According to an HRV study done recently on 300+ athletic & non-athletic people from ages 15 - 90 in California, the average value for a 52 yr old would be 60 in ithlete units, and 80 will put you in the top 2% of your age group!
Having said that, I have a couple of very fit cycling buddies who are in their early forties, and regularly getting scores of 90.
I think you are doing really well with that average reading, and I don't know how much further it can be improved.
There is a relation between the average ithlete reading and aerobic fitness (VO2 max), so after the competitive season, you can see how much the average goes up during endurance base building.

Hi Simon

ithlete measures the R-R interval (ie the time between the sharp spikes on the ECG) precisely in milliseconds, calculates HRV parasympathetic parameters rMSSD and Poincare SD1 (they are the same measure with a scaling difference), then log transforms & scales the result. I can't disclose the exact formula until the patents are published, but to give you an idea, an rMSSD of 25ms (SD1 17.5ms) will be 65 on the ithlete scale, and rMSSD of 55ms will be 80 on the ithlete scale.

The US study 'Relation of High Heart Rate Variability to Healthy Longevity' is available for free viewing at: http://www.ajconline.org/article/PIIS0002914909028525/fulltext
For those who don't have time to read it, the main conclusion is that: "healthy longevity depends on preservation of autonomic function, in particular, HRV–parasympathetic function, despite the early age-related decrease". That reinforces the value of keeping your HRV going up not only as an athlete, but as someone who wants to live a long & active life!

Hope this helps.