Bike base building question

Yes, trainer time is mentally tougher-no distractions, nothing to concentrate on etc. There is probably no real correlation to speed trainer vs road. Just focus on the HR workout, maybe dd some intervals to break it up.

Yes, trainer generally feels harder.  It's generally hotter and more boring.

It is usually better to go for a higher intensity on the trainer (but shorter time) than on the road.  Assuming you've got your zones dialied in, Z2 on the trainer is not normally a good use of time.

Bennet's suggestion of intervals is spot on.  Do 10-20x 30"/30" intervals, or 4-6x4'/1', or 2-4x10'/2', or 2x20'/4'.  These are just examples.  Whatever you do, you should finish feeling like maybe, if your pride really depended on it, you could do one more at a similar intensity.

I agree with other posters.  I would take it one step further and question if there is any use of a steady zone 2 trainer ride? Intervals are the way to go. You can do some long and short. For example for the 1hr and 30 minutes you could do 8 minutes in zone 2 then push to zone 3 or 4 for 2 minutes and keep repeating. This could simulate a hill climb every 8 minutes. This will break the boredom and build fitness.

Kevin

I think there is a benefit to both ways.  If you can tough it out on the trainer for increasing duration to build your base, that is great.  However, if you could add some intervals in there, it might be even better.  As the other guys said also, intervals will break up the monotony and make the time go by faster while sneakily building your fitness. 

Trainer Road has a good setup where you can calculate your wattage using your bike data (gearing) and trainer make and do workouts that show your watts.  And, because you have something to compete with (the line for planned watts), it is almost like a game.  Add some good music on Pandora and a big fan and you are set.

To your original question, I find it a lot harder to hold the same watts inside as I can hold outside.  For example, yesterday I rode outside and on one of my loops, I was holding watts that I simply can't do for more than 30 seconds on the trainer.  Something about seeing your progress up a hill makes it easier.

Don't attempt to compare your indoor speed to outdoor speed. They're simply not the same thing, because the resistance on the trainer does not match the resistance that you encounter outside (which, after all, is quite variable). But this doesn't mean that you should ignore your trainer speed and only pay attention to HR. If you set up your trainer consistently (and if it has continuous resistance), then you can certainly compare your indoor speed in one workout to other indoor workouts that you have done, and you can use that to track your progress. 

Most people will recommend focusing more on shorter, more intense workouts when riding indoors. The idea is that, unlike running, you can gain the benefits of intense training with few of the injury risks, so why waste your time riding in Z2. There's a lot of truth to that. However, I think that there are also real benefits to be had from the less intense rides. I find that when I'm doing more steady bike miles in addition to the more intense rides, then I'm able to recover more effectively from the harder rides. This is the same as we standardly do when running - the steady miles make us better prepared for the faster stuff. And it's the same thing that elite/pro riders do. But for most of us AG triathletes we don't have the time to train like pros and also fit in running and swimming, so we end up jettisoning a lot of the steadier riding.