How quick do you loose form?

If you're training with HR with a HR strap and you're on Strava, download the "Elevate for Strava" Chrome plugin.

This has loads of really useful extra Strava features, but one of the best things is it has a "fitness trend" feature which will use your TSS scores from your rides to show/track your fitness/form over time- this is really helpful for showing when you need to back off (if your fatigue is going up) but also shows how much fitness you'll lose over time. 

I'd say two weeks is where it starts to be noticeable - what you could do before without thinking will take more effort, and take more out of you.  You can go a week without noticing any negative change, but two weeks, you will be aware.

I broke my shoulder in August last year, and the combination of 4-5 weeks off then, plus the onset of winter means I haven't got back to where I was yet. I was in a good place then, and it becomes self-perpetuating (for me at least) when you get to a certain level, you want to maintain or improve that, so you make sure you ride every 2-3 days, if not more than that.  And you mix it up, a long slow ride, then maybe an hour of hill reps next time, then maybe a couple of sessions with sprints in.  

My recovery started with walking, I live in a beautiful area so literally 5 minutes walking from the house and I am away from buildings and in the woods.  I also noticed how easy/quick it was to start, no "special" clothing, no preparation, no having to have the right snacks and drinks ready, no charging of batteries for any kit, just normal cloths and trainers and away I went.  

I've now started running again, going twice a week with my 21 year old son.    Added to that, I have started being more careful with food and drink, and have at least got my weight back where it was before the accident.

I had a month off last year.  My fitness plummeted, took 3 months of training to gt it back.

I stopped celebrating Christmas yesterday (started it mid Nov) and got on Zwift for a quick 10 miles.

I'd become listless, not sleping as normal and putting on around 1 stone, so finally bit the bullet and decided it's time to get ready for summer.

The immediate impact of hibernation in that first ride was hitting max HR on a short rise that shouldn't be an issue and also for the time taken for HR to fall back as I wound down. 

At least it's a guage to measure the coming improvements.

I had a really sporadic year last year, going a couple of months between rides at points and only managed a few thousand km. the first couple of days are tough going but it comes back pretty quickly once you get some consistent mileage under your belt

Do something else. Is there a nice warm gym locally that you can join for the winter months? Good idea to mix up training in any event, different cardio exercises and 3 sessions a week of weight training to build strength.

Two weeks really isn't long. CW published an article about detraining:

https://www.cyclingweekly.com/fitness/training/detraining-the-truth-abou...

From reading widely, it seems that your fitness will return rather more quickly than it took to build in the first place. Joe Friel is one of a number of coaches recommending that for older athletes benefit more than younger ones by doing some higher intensity work through their off-season. Friel's book Fast After 50 may be of interest: summary / review.

TBH I'd not fret too much. You have plenty of riding under your belt so I'd look at training for the specific demands of those events. Hilly? Ride hills. Flat and fast? Group riding/chain gain, etc etc.

I don't know how long it takes to lose it, but since I posted my request for advice on this very site…

https://road.cc/content/forum/252535-how-am-i-supposed-motivate-myself

.. i have probably cycled into work four times at the most. 

In that time, I've put on a stone and am feeling even worse mentally.