You should definitely get the second edition. That said, here's some more advice: Your intervals are SHORT for the amount of rest you are taking. It takes two minutes simply to get to VO2max when you are doing an interval workout. Of course, your rest is just short enough that you probably finally do get up to VO2max somewhere near the end of the workout. You'd be better off either reducing your rest a bit (1:30 for starters) or increasing to 800m intervals. My guess is that you will find this workout suddenly much harder and will re-appraise your VDOT accordingly.
The 8k tempo is a much better indicator of your true fitness. You don't mention how hard you found this effort but I'm going to bet that it was somewhere just shy of your true T-pace. You should be able to hold your T-pace for 20 minutes of running with a feeling that the effort is comfortably hard. COMFORTABLY hard. If it is an exercise in pain, you are running faster than T-pace.
There are two key points: (1) T-pace is something you should be able to hold for an hour in a race situation. If you can't imagine running a 15K race @ 5:32 pace, you are overestimating your VDOT. (2) Daniels does provide a chart for T-pace workouts that are longer than 20 minutes and up to an hour. He's done so to acknowledge that there is some, that is SOME, benefit to LT when training at paces below true threshold. The longer you run at a pace below but near your LT, the more benefit you accrue. That said, all things being equal, 20 minutes at LT give the same benefit as 60 minutes at some sub-threshold pace. So why do the latter rather than the former? Mostly, if you are training for a marathon or if you want a break from track running or if you are coming back to running after a break and feel more comfortable at slightly slower paces for a while.