My glib and simplistic answer is to "do all you can do!" Indoor training is so freaking boring that I tend to ride harder than I would outdoors.

I would first set an "anchor point" at approximately the max wattage you can hold for a one-hour race (approx. 40k). Call this PD60 (for "Power-Duration Curve; 60 minutes"). In order to avoid the pain and suffering of a true max 40k effort, a good proxy is to do 2x 20-minute hard sessions on the trainer. It might take you 3-4 of these sessions to dial it in, so start by setting the wattage comfortably low on your first set, and raise it during the workout so that by the midpoint of the second repeat you're getting pretty sick of the whole thing. Next time, start 10 watts below that ending wattage and run through the whole 2x 20; adjust as necessary for another workout or two. Let's say that figure is 180 watts.

There's your wattage for your 1-hour session. 10 minute warmup, 2x 20 minutes at PD60 (3-4 minute mental break only), 10 minutes cooldown. Try raising wattage 5-10 watts every two weeks.

A 3-hour ride (yikes!) might be done as 45 minutes at 60-70% of PD60; 90 minutes at 75-85% of PD60; 45 minutes at 60-70% PD60. Basically, seek an average in the vicinity of 75% of PD60 and see how that goes.

These are just two of many workouts, and you can build variations on a theme by raising and lowering the intensity.

BTW -- to give credit where due, I derived much of the above from various things Andy Coggan has written. A very nice read is at:

http://www.cyclingpeakssoftware.com/levels.html#table2

Julian,

That helps a lot!! A very good starting point. Thanks for the help!
BTW any more on the HED bar pics?

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For one who has no objective, nothing is relevant. (Confucius)
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I have a tacx trainer as well - have you checked out the training schedules on the tacx website? Some of the interval workouts are watts related and you can download them in Excel and tinker with them as necessary. There are also some other resources eg RAMP testing etc.

Its work a look if you have not seen it but they may be too short for you - look under the support tab at www.tacx.nl

I use a TACX Basic. Can't imagine doing 3 hours on it - even with the most awesome movie on TV... perhaps while watching IMHawaii coverage.

Anyway - to answer your question - I did an evaluation test per Joe Friel's (tri-book) guidelines. Presuming you are used to training zones, you can figure those out using this test. From what I recall of this:
Do a 10-minute warmup. Start at 50-100 watts (set on the trainer with slope zero), every 1 minute add 20watts. Have an assistant note heart rate and rate of perceived exertion. At one reading assistant also notes VT ventilatory threshold (start labored breathing). End the test when you can't hold the wattage.

Estimate Lactate threshold by
~ Betw 15-17 RPE (scale of 20
~ Somewhere close to VT
~ Max power obtained * .85
~ Graph Power (x-axis) vs. HR (y-axis) - there will be a flat or dip in the curve, that will correspond to LT.

Now you have LT watts and HR. This is the start of zone 5. Zone 4 is approx 9 beats/min less, zone 3 is 7 bpm less, zone 2 and 1 below that.

Friel's book has a table for this, post your Lactate threshold wattage and HR if you want me to look it up for you, or you could swing by a bookstore and look at the tables.