Partner backed out of HIM relay- Should I attempt it solo?

Hello everyone! This is my first post here, but I've been lurking for a month or two (mainly trying to read every thread in the gear forum, but also dabbling in beginner, ironman, and nutrition). I'll try to make this clear and concise, but it will probably end up a bit long-winded, and for that I apologize. Any advice or insight I can get from this is greatly appreciated.

The short version:
I have signed up for the Duke Liver Center Half Ironman http://www.setupevents.com/index.cfm?fuseaction=event_detail&eventID=1188), and originally intended to do it as a relay with my neighbor. I was going to do the swim, he the bike, and then I'd do the run. Unfortunately, he will not be available that weekend, so I'm contemplating doing the entire thing myself.

The long version:
I'm fairly out of shape right now, but I have a strong running background from when I was in the military. At my peak, I was running at least 100-150+ miles/week, but that was 7 years ago. I did very little exercise between 2003 and last summer, when I started training for another marathon. Unfortunately, I pulled my calf around mile 20 of a training run in October, and stopped running until earlier this summer. A coworker heard that I did a few triathlons when I was in the military, and asked me to fill a gap on her relay team for a sprint distance race http://www.setupevents.com/index.cfm?fuseaction=event_detail&eventID=1177). I'm not a strong swimmer, but I handled the 750m in choppy water without too much trouble, but put in a very slow time even for me. That reinvigorated my interest in triathlons, so I made the decision to start training for an Ironman again (I was trying to train for one during my military service, and did all the individual pieces, but never put them together or competed much). I decided some good mid to long-term goals would be the New Orleans HIM in April, and then a full IM in November (I'm debating between IMFL and Beach 2 Battleship, leaning towards B2B due to proximity to me). I figured I could do a few tris or relays late this season just to get into shape and get past some of the stupid beginner mistakes I am bound to make, and start training intently next year. My partner backing out of the HIM relay is an inconvenience, but I think I might actually be able to pull it off myself.

Yesterday, I went for a "long" ride (this was only my third ride on the bike (Cervelo P2C) since I bought it last month, and before that, the last time I rode anything was around 2001), and did a ~40mi hilly ride http://www.beginnertriathlete.com/discussion/training/map.asp?routeid=107833) followed by a 4.7mi run/walk. (All of my recent workouts can be found here:http://connect.garmin.com/explore?owner=mwgemini). My average on the bike was 15mph, and I think my test ride is pretty close to the hills of the actual course http://www.beginnertriathlete.com/discussion/training/map.asp?routeid=84736, http://www.setupevents.com/files/2009BikeElevation.pdf). From a close examination of my usual run routes with the actual race course, I think the actual race is just a bit more hilly. I felt great after then run, and I feel great today. I definitely feel like I could have gone farther and faster (although not by much), but I would definitely be slow/weak on more steep hills.

My "plan":
I'm thinking that with my performance at the Nuclear Mile last month http://connect.garmin.com/activity/12161344), I should be able to put in a 45 minute swim time, especially if I'm wearing my new wetsuit that I've yet to try out. For easier math, if I can be out of T1 in an hour and average 15mph on the bike, that puts me out of T2 in less than 5 hours, which should give me 3 hours for the run. Even if I can't run it all (which will probably be the case given that I have not trained for the bike AT ALL), if I can average <13.5min/mi, I will still finish under the cutoff. I think I can pull that off with a run/walk, given my pacing yesterday. Worst case scenario, the swim and/or bike goes worse than I am hoping, or I don't nail the nutrition (which is my biggest question mark right now), and I end up getting pulled off the course during the run because I can't make the final cutoff. If that is the case, I will no doubt learn a lot about everything, and that knowledge will help me out a lot in New Orleans and B2B/IMFL next year. Best case scenario, I (barely) make the 8hr cutoff, learn a lot, and gain the confidence to shoot for a <6hr HIM in New Orleans and maybe a <13hr IM next November. Either outcome works for me.

My question: 
Should I attempt to do the whole thing, even though I have basically not trained for it at all, or should I stick with my original plan, and just do the swim/run (assuming I can find a replacement cyclist), or just skip the race entirely?
 
I appreciate any input/advice.

Thanks,

Mike