You need to increase your aerobic threshold by running long and slow (below 75% Max Heart Rate). You also need to increase your Lactate threshold by running 1-6 miles a tad slower than your 10k race pace.
These combine to allow you to run longer at a faster pace, or the same pace easier
No pain in the calf doesn't mean it's completely healed and strong enough for continuous loading. I've had a few grade 1 and 2 calf tears and have had them tear again when I was convinced it had healed (and that was with very short jogs)
I think you should be completely fit, healthy, and well prepared for a marathon, especially your first as you don't want the experience to be tarnished in any way.
I would have thought that long miles at easy pace (slower than MP) would be just as beneficial for HM training as it is in marathon training -in other words, training runs that develop your aerobic threshold