I would like to thank everyone for the extremely welcome input. It really is a great privilege to get advice from more experienced runners. I take inspiration from your experience, MidPack45, and although I may not have 2:50 in me, I do hope that increasing mileage will bring down my time.
If I take all of the comments and try to distill a message it is that doubles have a physiological benefit in and of themselves but are also an efficient way to increase mileage. And, of course, the ultimate goal is to get mileage as high as possible, without getting hurt. Since higher mileage is giving me a few minor aches and pains, it seems sensible for me to shorten the long run slightly (on the basis that frequent runs of 3.5 to 4 hours are tiring me out) and introduce doubles on most days of week. But I do take the point that longer single runs might be more appropriate for the races I have planned (35 miles and 54 miles), so may mix things up a bit once I'm more comfortable with the higher mileage and as race day approaches.
I can't resist a brief comment on the life balance issue that has been raised. Our system in South Africa is a bit different to yours and I am not an attorney. I'm what the English call a barrister. So, I'm totally self employed, which gives me a lot of freedom (for which I am extremely grateful). But I'd like to agree with those who take the view that running is worth pursuing, regardless of talent. In my case, running 8 to 11 hours a week has made me better able to cope with work and family life. It would be fair to say that it transformed me completely. I would happily run that volume even if I could only ever run a marathon at 9m/mile. The goal of reaching particular times is purely to keep things interesting for myself. Definitely not for fame or fortune (although, in South Africa, a 3:00 marathon for a 40+ runner isn't too