I'm not a gym person so I can't comment on that aspect but how much actual "weight-bearing road time" had Paula R done before the Beijing marathon? I suspect if you are able to maintain your fitness levels and perhaps go with a run/walk strategy you will be in a better position than many who toe the start line but do bear in mind that it is a combination of the impact of road running / time on your feet in a marathon that take their toll so you might not enjoy the experience.
I started running aged 28 and after 6 months of training twice a week ran 1:49 for a HM. Fast forward 6 years and I've just run 1:24:53 at HM. If I'd been more consistent in my approach to training I could have got to that point a couple of years ago and I'd like to think that I've still got a few years of improvement left in me. Like TT I don't think age need be a limiting factor - many of the best runners local to me are in their 40s. I do recommend joining a club. It helps enormously with speedwork and also gives you the opportunity to do team events.
I find that I really struggle with running for at least a week after giving blood but for some reason I'm unaffected when swimming. So I plan to swim instead of run for about a week after giving blood and I don't do any hard running sessions for two weeks.
Andrew, I agree with Karen and JEJ - run the half at MP (but you have to be sure that you won't end up racing). I ran a half at marathon heart rate 2 weeks before London last year and it was a real morale booster for me because it demonstrated that my target time was acheivable.
This week was a light week for me so that my legs are fresh when I go on the Algarve running week on Thursday.
Mon: easy 5 miles (didn't fee particularly easy though) Wed: hard 10 miles at the club (really pleased with this run) Fri: massage (couldn't believe the number of aches and pains the masseur managed to find. Sun: 20 miles in 2h 52
Total: a mere 35 miles but overall very pleased. Long run was one of the most enjoyable of this year. Looking forward to some warmer weather on holiday though.