I've tried a few and am finally happy with my Salomon Womens XT Wings 10+3 Vest. Worth noting that that with back packs the anotomical differences between men and women are quite important for fit . No matter what size your lady lumps are we tend to be narrower around the back.
Did this one earlier this year. It is well organised and as people have said it is easy to switch between full & Half Marathon. The second half is more sceanic than the first half. The only thing they couldn't organise was the weather, it was bucketing with rain and freezing cold.
Peds,I live in Colchester (Fingringhoe) and am also training for the St Peters Way Ultra. Slow and steady on longs runs - similar pace to yourself. Wasit you who was in the Essex County Standard the other week?
Message me and we can exchange emails as I'd be up for a few runs round Mersea or long runs anywhere.