As recovery runner said Mizuno do 2 shoes designed with forefoot strike in mind (they have the wave cushioning just at the forefoot). They are the Levitas and Cursoris with the Levitas being the more minimal shoe.
I have the Levitas and really like them, if you shop around they can be had online for £50, also if you like Saucony and want something more minimal then the Kinvara 3 can be found at the same price, they don't have specific forefoot cushioning but they are a great shoe for mid/forefoot strikers.
I am not the typical shape for a runner either, I have only recently stated doing parkruns so my pb is 21:01.
I was in a simillar position to yourself last year before my first half marathon, aimed for around 1:37 and actually finished in 1:34:04. I am not sure about the GNR course but would guess its fairly flat, if you are doing hilly 11 mile trail runs then 13.1 miles on the road will be grand.
As Seth says body shape isn't always an indicator, my GF's brother is 6''3 and well built but can easly put me to shame over any distance.