I am an avid Daniels guy and a somewhat accomplished masters runner.
I will say that a straight 20 minute tempo run and Daniels' T pace is among the tougher workouts I ever do. I have always had trouble with them, starting with the first one I did as a new runner walking on to his college team.
Cruise intervals, however, usually feel very achievable to me. The standard 4 x 2 miles at T pace with 2:00 rest usually doesn't start to hurt until late into the third cruise interval (although the first one sometimes starts out rough).
I cannot agree more with those who are saying to find a race and use that to establish your current VDOT and then use that as your baseline for training paces.
Generally though, if you are barely holding on at 8 or 9 minutes into a 10 minute cruise interval, you are going too fast.
Daniels likes to use the talking test to gauge effort: when you start getting somewhat deep into your T effort, you should be able to respond to a question if asked one, but you should not be able to maintain a conversation. I use the pledge of allegiance: If I can get out an entire clause of it at a time, but cannot recite it continuously, I can be pretty sure my effort level is appropriate.