Not sure I agree with all the Wool fans on here.
On a training run, sure, nice and cosy.
But for an elite Marathon race I think it retains too much water, adding a lot of unnecessary weight.
I think ideally all the clothes should be made of materials that retain as little water as possible. Something like polypropylen would be good.
I think a Brynje style fishnet long-sleeve (these are made with polypropylen) with a very light, form fitted windbreaker on top could be an optimal solution.
We use these fishnet base layers a lot here in the Nordic countries.
Because of all the holes, it retain very little water, and you also don't get that wet feeling on your skin, as the contact area is quite small. But it's plenty warm if add windproof layer on top.