Cake in your defence the bike is relatively flat and it is quite possible to go balls out on that course the run is undulating a bit not hilly at all tho'.
In the nicest way possible TNLN 7hrs for a half at your current fitness/weight will not be possible - 2016 would be the better goal. I would do the olympic and sprint tris next year and get confidence in doing that distance and meeting the training needs for that goal - when you have that under your belt move forward then to the longer distance.
Oxford Half done and dusted not a PB but given the lack of training I had a great day out with an old friend who did amazingly well given he has stage 4 bowel cancer and had his last round of radio therapy two weeks ago - lol the last three km he was dragging my sorry bum around the course my bad leg was not behaving well by that point and it was only mental toughness that got me round and knowing he was doing it much harder than I was that keep me from walking the last three km
13k slow run plus 2k WU/WD stinking humid along the canal - ate everything in sight when I got home my fault for not getting up at daybreak and then deciding the garmin needed to be charged and therefore the dishes and washing could be done and hung out
Should be okay for Oxford might be a good case of DOMS afters tho' my fault for getting sick and letting life get in the way of training!