Hadd sounds like a good place to start, especially as you've given yourself a sensible amount of time to prepare. You may need a bit of trial and error to assess your HR stats/profile if you're going to use it specifically for training, as it's generally easier to get HR significantly higher when running compared to cycling. As RicF says, the main task is to condition your legs for running; you're trying to get your running muscles to catch up your aerobic system.
Also I don't think there's any harm in keeping up a good volume of cycling, since it will complement running fitness, keep you aerobically fit doing an activity you're good at, and prevent you from doing too much running too soon, thus helping to minimise injury. (And if, perish the thought, you do suffer any running injuries along the way you've got an excellent cross-training activity to keep you fit.) Gradually substitute more running for the cycling, perhaps building up from three to five times a week, whilst slowly building up mileage. Don't worry about specific marathon training plans till well into the new year, but consider doing some warm-up races along the way.
Some of the comments on the Sky news article are hilarious. There are a number of people that that actually believe that his speedy second half was still possible.
"Mo Farah is a middle distance runner not a long distance runner so its not that strange to think somebody was quicker than him."
Mo Farah, the British half marathon record holder? That Mo Farah? Seriously, it's perfectly OK to be ignorant about running (I haven't got the foggiest about fly-fishing...) but why would you even bother to comment on something if you haven't got a fucking clue? I suppose that's how the internet works though isn't it. Everyone has an opinion on everything.
CW - Ouch! Tough luck fella. It's a miracle you managed to squeeze in a quality marathon in between your injury woes, but I hope this one is short-lived.
HR - Thanks, much appreciated. Feeling a lot better and I think it's on its way out but still not completely cleared up, and looking out at the miserable weather this morning I decided to take another rest day just to make sure. Should be back on it this week, I'm sure the rest will do me good. I spent a lot of yesterday exercising my, er, exercise envy either cleaning my bike or plotting routes on my new Mallorca cycle map. (I was considering getting a flashy Garmin satnav for the bike but decided old-school is the way to go!)
Even that looks odd... His Gun Finish Time is quicker than his 20km split time...
That just appears to be the way they've listed the split times, with the final gun time listed as the chip time. I checked this for someone else I know who ran the race. So, a legit time, just a crap one.
Maybe he was carrying a really heavy charity bucket?