Sounds like you've gotten good advice so far. I also think the power meter is not needed for this. I had a coach for a while and she was able to use information from my Garmin, including heart rate and cadence. However, to be honest, I didn't feel like I got much faster... but it was interesting trying the intervals.

I have done one self-guided tour with rides of mostly 40 miles per day. Just doing a lot of regular rides at that length was good training. You might not be able to replicate 60 miles / day for multiple days but I'm sure that once you have some basic cycling fitness for 30 miles, you can up that to back to back rides on a weekend with at least one 60 mile and one 40- 50 mile ride if you can't do 60 miles for two days. That should give you some idea of how you'll feel after two days.

The tour sounds great- where in New Mexico? How much climbing will be involved? 60 miles flat is really different that 60 miles with 5000-7000 feet of climbing.