Mixing HR based training and 'Normal' training

I am following a marathon training plan via a thread on this forum. I am happy in that plan and I feel that it will get me over the line at the VLM2013 in a good time. It is a traditional plan. It is based on mileage and the paces I have been recommended are too high to build endurance I think i.e. I think i will be operating well above 70% max hr.

I don't want to deviate from that running plan but I am concerned about building endurance for a September 2013 IM. I was wondering if I do low HR cycling on the days I am not running whether that would help? Would the endurance I develop from the cycling help me in my running at the IM too? I know they use different muscles. Would the fact that on my running days my heart rate would be much higher spoil the low hr work I did on my bike? 

I'm a bit confused as you can probably tell from the words above. HELP! Please.

Tortuga - your easy and long runs on your marathon training plan should coincide with the general aerobic work you'd do for long distance tri training (Don Fink's Z1 -2). If anything I'd suggest dropping any recovery runs and swapping those for rides (remembering that your bike HR zones will be different from your run ones). I have consistently dropped my running PBs on a diet of Z2 runs and rides and three swims a week.

Of course if you are really worried/targetting a very fast time at VLM you could concentrate on the running with some swimming until then and become more tri specific thereafter.