BrokebutTri'ng - you are probably overall fitter due to the cross training of Tri, but you wont run an optimum marathon off it regardless if you can put more intensity into your running or not. Top marathon runners run, they dont swim/bike/run. Running intensity rarely features in a marathon build up for me, but a bit more run frequency and the associated miles make a big difference to how my legs cope on the day. You might be a bit nearer to the pointy end of marathons than me though.
Sweetfeet - you couldnt turn that VLM place down then, taking it as a training run will probably enable you to enjoy the day more, taking part in VLM is teh closest I'll ever get to a Tour De France type atmosphere on race day. B'mth marathon noted by me too. I already have a date with Sandyballs again though.
Seren - I like that marathon plan, slow running miles topped off with Parkruns or XC races. Nearer marathon time if you can make a couple of your running days double days with something like another 5M run this really helps your running legs. I only run 3 or 4 days a week in marathon build up but they are all commuting doubles apart from the long run.
I have to bike early mornings so apart from some MTB commute miles then I'm pretty much turbo bound, with the odd weekend ride if its not icy early or flooded (more apt at the mo). But Turbo is good for me as it works me hard.