Normally I wouldn't specially recommend them for a long time, but how about some Goretex shoes, maybe coupled with some shortie gaiters if there's a lot of wet long grass. IME over a lot of trips the goretex membrane will keep your feet dry for a few hundred k but eventually it starts leaking where the upper creases, long before the shoe wears out.
Looking on a single trip basis, it becomes a toss-up whether the membrane keeps more sweat in than it keeps water out, but given the length of the trip you propose, it could be worth it. I'd change into something else at work and let them air-dry somewhere, possibly with a boot dryer if you will do this even on rainy days.
The question is: if you run a half at flat heart rate, does you pace stay flat, with an equal or slightly negative split? For me there is a slight fade, which I offset with say a couple of % either side of 88 % in thirds, IIRC. I don't often run a half, but here's my half pb from 2011 with a trending up, mini-marco HR. I could have been too generous here, since I seemed to have quite a bit left for the last 3 km
Edit: here's a better one from last year where I aimed at 87, 89 and 91% for a trail half.