Thanks to all for your replies. Runloz - I hope you are able to get back running very soon; your story puts my (hopefully) temporary "downtime" into perspective.
The message is very clear - come back slowly and be very patient. I've also been looking for advice on the internet and the "Livestrong" website recommends a week of 20 - 30 minute walking followed by (assuming no adverse reaction) starting to run slowly for 25% of your pre-illness times (i.e. if you usually ran for an hour steady, start at 15 minutes).
At first, I thought that sounded unnecessarily cautious, but I think the danger of pushing too hard too soon is such that it's better to proceed too slowly rather than risk a relapse.
I'm planning a 30 minute walk tomorrow and I'll take it from there.
My specification of "over 50" was a bit arbitrary, but I'm aware that recovery from even minor injury or illness takes longer as you get older and I didn't want too optimistic a scenario !
I'll certainly follow your advice about taking it easy. I've mentally "pencilled in" late March/early April for returning to racing (giving me a couple of months + to get back to full fitness) but I'll have to see how it goes.
I'd appreciate anyone's personal experience of getting back into regular running after pneumonia, especially anyone over 50.
I'm currently recovering from a nasty chest infection, which my GP now thinks was probably pneumonia (I had seen another emergency GP when I was treated for the original infection). I've basically been "out of action" for nearly 4 weeks and although I'm fortunately now in the recovery stage I still have some lingering symptoms and am doing nothing strenuous yet.
Before this happened, I had been running between 30 and 35 miles a week for about 20 years, including regular racing and speedwork and had been lucky with illness and injury.
While I know I will have to proceed very cautiously when I do start running again, I'd welcome any first hand experience of how long it might take before I am roughly back to where I was, as it's always good to have a (realistic) target in mind.
You've still got approx 14 weeks before the London Marathon, so assuming you can get back running within the next fortnight, I'd say you still have plenty of time to get the training in. I'm not a marathon runner myself, but the general consensus is that it's building up the weekly long runs to around 20 miles that is key and you should still have ample time to do that without rushing things too much.
Even if "the worst comes to the worst" and you don't have time to get properly ready, there are plenty of other marathons you can target, for instance during late summer/early autumn.
Although you shouldn't try to jump straight back in and carry on from where you left off, the training you did before Christmas will still be "in the bank" and will help you get back into more quickly than if you were starting from scratch.
Good luck and in a few weeks time, it'll just be a memory.