Posted: 06/10/2015 at 14:06
I'd cancel it.
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.