Here is another workout for you. Yesterday, I took my time trial bike to an island in San Diego where one can ride 4 mile laps of flat terrain. This time of year is strength and adaptation when preparing for events in the late spring. And I am getting over a cold.
I decide to do 2 laps of low cadence work in the aerobars. This means riding at 55 to 60 rpm and I my setup is a 54/44 11/23 cassette. I plan to do two laps in the 54/11 combination. I have to come up with a warmup routine for this effort.
I do 15 minutes of z2/z3 and then 3 seated 10 second accelerations and 1 standing acceleration with 1 minute RBI. I spin for a couple of minutes and then do a hard 30 second high power higher cadence. I spin again and then shift into 84 gear inches roll to a stop a punch it seated i.e a seated start about 10 seconds. I spin a little and then in 96 gear inches roll up to a stop and do a standing start. That completes the warmup and then I did two laps of tempo power low cadence laps. Total time was 1:05. I worked my ATP PC, glycolytic and aerobic systems.
If I would not have been sick, I would have done another two laps that would increase my time maybe another 25 minutes.
The goal is to get your juices flowing with an idea of a way to work all the systems in a short period of time and gain some fitness. You could skip the sprint front end that I did and just do 2x20 of tempo or just warmup and and do some sprints and starts, cool down and count it as a strength day.
There are a lot of ways to generate aerobic fitness and strength without a lot of time.
Having said that, if one want to spend 6 hours in the saddle then one has to practice sitting in the saddle for hours. YMMV.