I'm in a suck place. I started riding in 2016. At that point I was 31/32 at the time. Riding like 35 to 40mi a week. In 2017 I picked up the time and started doing some structured stuff.
I never had the chance at a big build early on like some folks have had.
Only shots are lining up a big gravel grinder in fall/winter and maybe a long fondo in "season" for which I can get some "honey please" time to get in some Z1/2/3 riding volume.
Otherwise, it's hiit workouts for me.
It's just what I can fit in. I run sometimes for impact workouts. It can be fun to attempt a duathlon or something if your bike pace is good.
Gains are hard now that the easy early gains are over. Gains that would be easier with a base.
If you're on less than 7 hours a week, HIIT it is.
The limiter also is that for those easy rides to matter they need to be long duration.