I currently track fitness using Garmin Connect, Xert, Strava and Intervals. Garmin is telling me I need 70 hours of recovery, Xert says I am below a continuous improvement level, Intervals is saying my fitness is fresh. These parameters have usually tracked fairly closely until the two weeks. Garmin is also reporting my activities as balanced between Lower and High Aerobic and slightly less than optimal for Anaerobic. Two weeks ago, I bonked on a hard ride that was at the end of 4 days of hard riding. I rested for 3 days, did a short punchy.ride, then basically a 18 mile TT over flat terrain with no wind in 52 minutes. FTP went up by 30+ watts. Xert is saying I am almost back to a training level that is improving fitness, Garmin says I need 70 hours of recovery, and Intervals in Fitness is saying I need to pick up the load if I want to continue to improve. In this same span, Garmin is reporting a drop in VO2Max from 45 to 42. I’m generally very competitive in my age group (79) and ride between 150 and 250 miles weekly. I have been focusing on trying to make sure I have adequate recovery as Intervals is showing I was in the High Risk zone for a good part of the Spring (before I found Intervals). Trying to understand how to interpret this data and train optimally.
Don’t know xert too much (just used a trial for some days) but:
Garmin shows recovery time to next same intensity workout, meaning you can ride earlier but it has to be much easier. In those 70 hours you can ride twice for example at z2 and for shorter time. That way you can maintain your weekly load target and if you plan your training you probably will end up with 2 hard session per week at max during the season anyways
Bonking can be fitness but can be just nutrition. But 4 days of hard riding sound weird and pointless. Watch TDF, even pros can’t ride all days hard, if they attack on one stage, the mostly ride easy next day. Ofc you do what you like, but the idea of training (and hard rides) is to give stimulus to your body and give it time to recover and rebuild. Or in other words, 4 days of hard riding is just in your head, in reality it was more of 4 days of medium riding at best. Reason why people mix easy rides with hard rides is just to make those hard rides really hard and to do that you need to be rested.
4 days of hard riding and 3 days of rest could be 2 days of hard riding with 3 days of easy riding inbetween (with one recovery ride) and one easy ride after second hard day to build some fatigue resistance plus one day off to reset from the week and rest completly. Effect would be like 3 times better.
You were rested for 3 days, you got higher ftp, nothing new bc you sound like an overlly fatigued person (ambition to training over being smart, nothing against you ofc)
Generally you want to pick up load to improve, it is called periodisation. But you can pick up load in many ways. Most of the time you do it during the winter with long easy rides. You prevent fatiguing and build aerobic engine to support hard intervals/rides during the season where you lower the load slightluy during your peak to have best legs etc.
To train optimally you could plan your whole next season (it is a good time for that now) put all your planned workout into calendar in intervals to see your future fitness and check if your ramp rate is not too high (6-8 max probably). And with such planned season you track with garmin and check your stats on a daily basis in garmin.
watch all parts of this classic and feel free to ask more 
PS. Like your passion to cyling, just add some brain work on top of that and you will be gucci
Your explanation of the Garmin algorithm was most helpful and provides a logical explanation for the disparity between advice from Intervals and Xert. It was during the 7 days following that Garmin ws recommending 70 hours of recovery while Intervals and Xert were suggesting a more vigorous regimen. During the periods of heavy activity (relative to my normal activity) I tracked the fitness and fatigue level in both Intervals and Xert. Neither showed me to be in the overtrained zone. The bonk was most likely from nutrition, not exertion. That day the group plan was for an easy two-hour ride that turned into a hammerfest of three hours on a very hot day. The heavy sessions prior were to apply maximal stress, ahead of a recovery period for a grand fondo. Thanks for your feedback. I will be looking to use Intervals during the winter where a more disciplined series of activities can be performed using Erg Mode with the smart training rather than public roads with traffic lights, stop signs and vehicles.
Your explanation of the Garmin algorithm was most helpful and provides a logical explanation for the disparity between advice from Intervals and Xert.
I use Garmin and I like it but must say the info about all the metrics is all over the place. If you dig deep you can read about it on garmin website but I think Garmin lacks something like a good explanation how to use/read their watches. Or in other words I have a strong impression looking at Garmin that they are like 20 desk office where those 20 guys/girls who work there are mute and have no connection with each other at all. And to learn about Garmin you have to visit all those desks, ask about a function this desk is working on, but keeping in mind that part of it is overlapping with another desk etc.
Neither showed me to be in the overtrained zone.
not going into much details, but at least intervals.icu is best used to plan bigger picture and chronic fatigue. It is mostly the build phase where you can go into overtrained zone. During the season where your activities are more or less constant (you reach your max weekly hours limit and reach your weekly fatigue limit too), fitness graph will stay constant too. From technical point of view it is just your 42 days avg TSS load. For much more closely tracking your daily training readiness you can use garmin (load graph and all other garmin metrics they work just fine) Garmin uses HR to calculate load and it sometimes gives much better idea of the load. My example from the other topic: 150km ride is twice the load of a 5x3min vo2max intervals when looking at TSS score, but Garmin showed higher Load for the intervals session, and my RPE agreed with Garmin not TSS.
The heavy sessions prior were to apply maximal stress, ahead of a recovery period for a grand fondo.
body needs 4-8 weeks to adapt to a high level stress, so if you plan a hard session before a grand fondo keep that in mind, maybe it is better just to relax and rest well.
I will be looking to use Intervals during the winter where a more disciplined series of activities can be performed using Erg Mode with the smart training rather than public roads with traffic lights, stop signs and vehicles
As I said earlier, you can just fill the calendar like months ahead with your roughly sketched training plan (activiteis like 1h z2, 2h z2, reco, tempo intervals etc.) and you will get a nice view on your fitness graph.
PS. The problem with Xert, Trainerroad etc. is mostly due to lack of planned recovery weeks and overall load being to high . Like they are good during the season but outside of the season when you naturally want to rest and prepare for the next season they just keep pushing hard efforts on you. At the end your fitness is pretty high but it is far from what it could be with more rest and annually periodisation. (you can keep fitness at 60-70 for a whole year probably, but 120 just for couple of months at best).