Some interesting points made about the importance of an aero position.
My actual reason for my training is to make my regular cycling easier, not especially faster.
The advantages of "Aero" for Audaxing are dubious to say the least when it's possible to ride successfully at 15 km/hr. I can't say I saw many, of the thousands of riders I have seen, riding in the drops even.
Comfort was my only concern when I Audaxed, the 2nd thing on my list was a distant 2nd and it was more likely to be food-related than speed-related.
Clearly being "aero" matters more the faster you go and, at Pro speeds around 60km/hr, it matters as much as anything else (let's say 100Watts to cycle at 15km/hr would mean a huge 400Watts at 30km/hr and a superhuman 1600Watts at 60km/hr UNLESS you sorted out your aerodynamics.)
EDIT. Wiggo required 475 Watts to ride at around 55Km/Hr and win the TT Worlds. You don't get much more aero than Wiggo on a TT bike but 475Watts for an about an hour is still HUGE and beyond most mortals. That sort of power in that much of an aero tuck are normally mutually exclusive.
My average speed, of my mainly social rides, hasn't really increased with my power, it just got easier (I rearranged the Lemond equation). That was the goal, to shrink the hills. It's nice that i can really dig in and put out some power if I want but I tend not to. I leave all the pain in the WB studio.
I suppose that next summer I may try and target some of my regular Strava segments but, like Time-Trialling, equipment and conditions play a huge part in the results. I have a faster road bike now, the wind may be on my back..etc. Wattbike is the only accurate measure I have of my improvement (or not).
An FTP test, resulting in a Power:Weight metric, is a perfect "Time Trial" in my opinion, it's independent of equipment or conditions. A level playing field.
On that basis I have been beaten into 2nd place by a middle-aged woman with a P:W of 4W/Kg whereas, out on the road, on a typical TT, I know I would beat her with my power and body weight (especially on a windy day).
I suppose the question should be: Do you want to go faster into your mid-50s or do you want to make cycling easier in your Mid-50s?
If you just want to go faster then buy a Triathlon TT bike, some carbon disc wheels, an aero helmet and a skin-suit. They really do zip (Zipp?) along, scarily so.
If you want to make it easier then do some structured, focussed, training and it will get easier out on the road.