First IM Distance race done! Did I hit "The Wall"?

Did my first IM race last Saturday, Vineman in Sonoma.  It is a very well run race on a challenging course in a beautiful location.  Much to my surprise I ended up with a podium spot, 3rd in my AG (M55-59)!  It was an awesome day and I loved every minute.

The temp turned out to be much higher than in most years, passing the 90 degree mark before I got off the bike.  I'd been watching the weather so wasn't surprised, and backed off a bit on the ride to save a bit more for the run.  Kept in lower to mid HR z2 on the bike, nutrition and hydration were right to plan (232 oz of liquid between H20 and Infinit, overall calories consumed 2260) and felt strong when I got off the bike.  The run started much as expected, legs trying to go too fast taking a conscious effort to stay slow, and a bit of a struggle to keep HR in z2.  The course is 3 laps with a lot of rolling hills with a couple of significant ones.  Early on I accepted I'd be walking the hills if I wanted to keep HR in check, and that's how it worked out. I had a 12 oz carry bottle I kept filled with ice and water, consuming about half between stations and using the other half as cooling on my singlet/head.  Also dumped a up of ice down the front and one down the back of the singlet (had a race belt on that trapped it).  Nutritionally took in a gel every 3 miles and supplemented with banana or cookie or chips as I could tolerate.  Took saltstick capsule every 5-6 miles. 

HR was staying mid to high z2 until about mile 8.  I checked my Garmin then and saw it had dropped to a z1.6 even though my pace had slowed slightly as well.  For the rest of the race try as I might I just could not get it to go back into z2.  I tried gels with caffeine, switching to cola on alternate aid stations, etc.  I was torn between forcing a greater effort to push it up, but also fearing that if I did so, I could potentially jeopardize the whole race - and here's where the inexperience shows.  I've never "hit the wall" in any of my marathons, and was feeling like I'd just experienced it.  In retrospect, was it "the wall", or had my HR just settled and I should have just pushed harder? 

Did I mention it was hot?  I've never been in a race where I've seen pretty much everyone walking the hills and many the flats, but that was the case.  I came over one hill and felt like I was in a zombie movie.  I had at least 20 people shuffling up the hill, open-mouthed, glassy-eyed toward me, all breathing heavily.  There were two over on the side, bent over puking, at the bottom of the hill were the paramedics tending to a guy laying off the side of the road.  During my first lap at about the halfway point a guy on a bike comes up behind me and says, "just to let you know, there's a runner coming up."  I'm thinking, "Dude there are runners everywhere here, what's the big deal?"  Well as it turns out it was the race leader on his last lap.  It took him at least 5 minutes to catch up with me and he was running just slightly faster than my pace, and he was hurting badly.  He got about 25 yards ahead of me and stopped, bent over hands on knees and stayed that way until after I passed him.  He passed me again about 5 minutes later, made the turnaround and headed back.  I caught up with him about 4 miles later as he had pulled himself from the race with only 2 miles to go.  It was brutal out there.  But oddly, the last lap was the easiest.  I was able to run longer between walk breaks, even up some to the hills I'd had to walk in the previous laps and increased pace 45-60s/mile.  So, Wall or just too timid?