Many thanks to everyone who has replied, it has been very helpful. I have basically taken richardsimkiss's advice and added an extra 4-5 mile run at a "sprint" plus 6-7 miles at pace and an 8 mile plus long run giving a total of about 20 miles per week initially.

I'm also going to extend the long run towards 10 or 12 miles over time - when I'm used to the extra weekly run - maybe even bringing a HM within reach (target time 2hrs?). I am trying to fit the swim and yoga in between and I'm leaving the stopwatch alone for a few months until I feel stronger, as it was spoiling my enjoyment of running. I'll be enjoying the long run much more running easy rather than against the clock.

I have always trained at the race pace of 8min30secs - 9mins per mile so I realise now that I need to slow down, enjoy more varied and longer training and concentrate on maintaining a firm steady pace on the day.

Fair play to you regular runners going 5 or 6 times per week or more but that's not for me. Hopefully the new training plan and some cycling in the spring and summer will build the stamina and an aerobic base to break the 90mins barrier for GSR.
I can recommend the Great South Run as a well organised fast flat run at a good distance - but watch out for a killer headwind on the seafront over the last two miles!