like the majority said everything has variables, however I have been riding since 1976, and before, it was cadance, then computers with cadeance and then average speed, now power meters.
i am 58 average about 120 -150 miles a week sometimes more sometimes less and yes I still use my average speed as a gauge and personally don't see anything wrong with doing it that way.
I also do hill sprints working my gears and large ring to the small ring and yes that builds a good power base and sprint capability

Ex: yesterday I did 30 miles about 5 mph wind with some rolling hills thrown in 1.39 was the time average was around 21mph
last thursday do too snow I didn't ride over the week-end I did 52 miles 3.05 was the time @ around 17 and change.
last tuesday 25 miles around 1.55 time easy tempo ride in my 39 ring.
Basically it dpends on how you feel and what you want out ofthe ride @ that time.
In the long run build a base of miles then concentrate on speed, always enjoy theride though