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NyoGoat - I'd go with a bike computer and a chest strap heart rate monitor. If you already have a running watch like a Garmin Forerunner, just get the chest strap. I'm pretty sure you can find a bike mount for the watch on Amazon.
As Carmichael points out in the book, the time-crunched program prepares you for events shorter than three hours. For longer events, you just need to add some longer training rides in addition to the intervals.
The gadget/no gadget discussion is a perennial argument among cyclists. I rarely ride without power, because my bikes have power meters.
The numbers become your master only if you let them. Hitting the numbers is important during a structured workout. If you don't hit them, it's important to know why. The overriding issue, though, is that you still got to ride your bike! RPE is important, but it's heavily influenced by external factors like life stress, illness and such.