My easy long run for half marathon training is at 30 Kms??

I cant imagine that you should be running 31K and 4:20 minutes in prepping for half a marathon. It's normal that you have long duration runs but i would say 25K max to be honest almost 32K is stretching it. 

On one hand I say good job doing the full distance. On the other hand I think you’d have been fine going an hour and a half out and then heading back.

In regards to your last sentence the opposite is true. A faster runner can afford to do longer runs since they can go farther in the same amount of time. My preferred marathon training plan is Hanson’s which is the one that people talk about as the one that only has 16 mile long runs, while most people think that you need multiple 20 milers to prepare for a full marathon.

But if you go back to what I wrote earlier about the length of long runs in terms of time 16 miles at 11:25 min/mi is 3 hours which isn’t an uncommon pace for a lot of recreational runners planning on going that distance. Pros on the Hanson team will run farther than what’s in their consumer plan because they’re faster. Personally I take my last 2 long runs to 18 miles because my easy pace when I’m at, or close to, marathon shape allows me to do that distance in around 3 hours. Even in the Jack Daniels plans it will give long runs as “X distance, or Y time. Whichever is shortest.”

Anyway, good luck with the rest of your training and I hope you meet your goals when race day comes around.

Depends on the person/plan/fitness.  Hard for someone to tell you specifically without knowing your training.  Personally I have always ran my long easy runs, if hill is super steep and running would put me over 'easy effort' or heart rate... then power hiking is just fine!  But if in a couch to X plan... then sure stick to the plan to slowly build up to running fitness.  Once you have running fitness (can run a few miles or more and/or have a good base of running regularly) then fast walking isn't as beneficial as running for fitness and improvements (heart rate will drop producing less training stress, mechanics are different producing less running economy improvements, muscles are different... you get it).  However - for a long run, if a 15mile run will put the hurt on your legs... some short fast walk breaks can keep the aerobic engine going, while allowing form to remain better as fatigue can reduce a bit during later half off run.  However, still a good idea to get some nice long fatigue inducing long runs in to provide the stress and confidence needed to run the race distance.  

Could see that once you go from a 10mi long run to 12mi... maybe first time run/walk (20% walk) then repeat at 10% walk... then repeat 5-0% walk.  

For me I usually only use walk intervals mixed in at the end of a long run when I mixed up distance and have a mile or more to run to get home still, then i'll walk/jog the rest.  Or when recovering from injury and I want to get the aerobic base training in with less stress on legs.

Elites do 32km a day for 5K races