Build phase is crushing me...

You're running too fast. (you're basically outrunning your fitness)  You want to be able to hit every interval at about the same pace.  If you can't, then you have to adjust your pace.

I might walk a little in the recovery, but usually it's a really easy jog, often well behind even an easy run pace.

Also, as was mentioned, the hard ones need to be hard, but consistently so across the board. First few, last few (and all between) should generally be about the same pace to just slightly faster on the later ones. Note that's for pace as HR probably isn't going to be the same, peaking a little higher as each interval is ticked off.

So are you saying that you can basically go the same distance with each interval?  Because if you can't you are kind of mixing apples and oranges. I can run for 30 seconds and jog for 30 seconds....but I'm going to get slower each time I run if I outrun my fitness.....the fact that I can still go 30 seconds is irrelevant...and not helpful for fitness building and getting faster.

If you want to go by just HR and your HR is spiking past some decent recovery in the 30s rest then you have to slow down. 

If you are running (for example) 200M in each 30s interval and your HR is creeping up you are fine....because you are hitting your interval each time.

The thing is, you aren't really doing "interval work"  with what you are doing.  It's more like a very short fartleck run.  If you want to do intervals, figure out what distance you can go in 30 seconds with your HR recovering to where you want. That's your interval.  Over time, you can increase the distance of your interval or add intervals as you get more fit.  It's easier to manage than what you are doing IMO.

I would recommend just slowing everything down.  Try 8:00/mile/ and 12:00/mile and see how you feel.  The idea is to work a bit harder, but not to the point where your next workout is affected because you went too hard.  Play with your times.....you will find that spot where you are working harder but able to extend the work without feeling "way too exerted" as you hold the pace.

Yeah, that is the idea, but not to the point that it feels "way too hard" or it's "crushing" you.  And I agree, that IS a lot of intervals, another reason I doubt it's meant to be "all out" or even 85-90% like you would a track interval workout.

You still have to work at a place where you can hold each interval in the workout.....either by time or HR....once it starts going too far north you might as well just stop the interval work......nothing good is coming from it.

Think many getting into it will need to redefine what hard feels like, but also with backing off some at first to learn how things work. Learn it over several attempts. See how much soreness comes in over the next several days first. Will still get some benefits and be more sure of not getting hurt.

Something like 30/30 can be used to help introduce VO2max type workouts as well, since it's never going for all that long. It might go to 60/60 soon and then out to 3-5' in duration.

HR during recovery in these may not drop all the way back down, and that's part of the point of this pattern. With the intervals being short, the effort would have to be much higher to get the oxygen processing up to really drive that adaptation (the goal of the workout). But doing that would limit the number of reps that can be done. The pattern is a balancing act of work and recovery to give a little break, but still stay close enough that the next interval be back up at that more optimal level fairly quick.

The longer 3-5' ones have more time sustaining that level within each interval and work more independently of each other. In that regard at least. Legs are still going to tire more after each one.

But again, don't try to knock this out of the park right off the bat.

ETA: 18 is a lot of intervals, but at only 30" each that's a total of 9' working. Mike's 4x4' example is 16' working. So it's a lot because they're rather short.

Nice.......well done.

One of our typical work outs when I red shirted on the Track Team at a D1 University my freshman year of college was 20 x 200m intervals.  Our coach had us take turns setting the pace.  If I remember correctly the leader had to be around 27 seconds and the last man (who was usually me if I was not setting the pace) had to be under 29 seconds (if the last man didn't make it we had to redo that interval so I was always nervous when I was setting the pace but I don't think we ever had to redo a single interval ).  I don't remember our rest interval but it was more than 30 seconds.  I think it may have been more like 2 minutes and yes we walked and stretched.  No running. So yes it is common to walk to bring the HR down, but it depends on the length of your rest period how hard you go.  We never wore a Heart Rate monitor nor worried about anything but catching enough of a breather to be able to hit the next interval with the last man under 29 seconds.  If we hit the interval then that meant that our recovery was sufficient. :-)