First off, great race report! I really felt like I was there.
Gosh, just reading about your back issues... how disappointing.
You have trained so hard and you are so focused... and then for it to "go to hell in handbasket" on race day... dang it.
BEFORE those back issues though, you were rocking it! You first run was fast and your bike too. That's something to be proud of!
2nd to last in a field of really seasoned athletes who are FAST runners... is something to be proud of... especially considering you had back pain/issues. I bet they didn't have any of that to deal with.
Next time, if you don't have back pain during the race you will certainly do better.
Otherwise, it looks like you were doing a great job before that back of yours took over. Be proud of that!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"