This is just my opinion RV...but I think it was way to soon after IMmoo to do a marathon. Especially a Marathon where you had time expectations..perhaps if you had said "i'm doing it but just for a finish" and lollygagged the whole thing.
Most recovery plans for IM have you doing nothing serious for three weeks to a month.
One suffers a system wide depletion after an event the length of an IM. Even though you feel "ok" you may have no reserves...nothing deep down to pull from. I've done several long events after my IM's and have felt worse at the end of those than I did the IM...It takes quite awhile to spring back...

I've never done an Ironman (yet) so I can't say too much about your recovery needs after that, but after every single marathon I've done I can't even try to run hard or even that long for at least a month. And that's after only a marathon, not a marathon after a really long swim and bike, so I'd imagine that your legs were really nowhere's near ready to give that run another go. Don't be too hard on yourself, you finished in what isn't a terrible time for someone in your place. But go give yourself the real recovery you deserve. I try to stick by the theory that you really shouldnt run in the first 5 days after a marathon and need at least one easy day for every mile you ran before you can run hard again (I've had coaches that tell me this applies to 5k's too). So give your legs a break! And congrats on finishing.

Thanks for the learning experience - better you than me ;) . I had originally thought of doing same - Lakefront after IMWI - but I haven't run a step since IMWI and couldn't imagine doing a marathon. But kudos to you for doing it.

Ya, funny how fast your perspective changes.
Now I have learned that while I don't need to be afraid of the distances, I still need to be respectful of them.