I say do it. You are in a similar position that I was in entering my Oly 2 weeks ago. I KNEW that I could not finish without some amount of walking. I just didn't know how much walking I would be doing. YOu read my race report, so you know the end result.
After doing the Oly, I entered the 10K this past weekend. TO be honest with you, I am far MORE sore this weeek after a measly 10K than I was after the OLY. WHy? Because for the Oly, I had a plan for my run/walk. I knew how much I could do, and I had a preset plan
(run 10, walk 1, repeat
). I kept this up as long as I felt good. THen I increased my walking intervals. I finished the run at an almost 15min/mile pace. I was sore for a few days, but was able to run without pain or fatigue 3 days later.
Fast forward to this past weekend. 10K should be a piece of cake after an Oly, right? Well the difference was that I let a friend talk me into running the whole thing. His goal was not to walk at all, whcih to me was moot, I just wanted to do a 10 K again. I ran the whole thing, ran throguh the knee pain that developed in mile 4 and am still sore. In a way, I am trhilled to "prove" to myself that I could do it, but to what end? Now my knee hurts, and that's a precedent I don't want to continue.
Bottom line...stick to your plan, do the swim & bike, enter as an Aquabike if the race is sanctioning one. If you feel like you still have energy, start the run, but have a plan. Don't let yourself run till you hurt, take walking breaks before hand.
In my case, with the Oly, the 10K route went past teh transition at teh 5K point, so I knew I could always bag out if I needed to.
My friend that I ran with is a great friend, but I don't think I'll be sharing my race plans with him, as he always "guilts" me into doing more than my planning calls for. But I digress...
DO IT!