Hannah,
You will have plenty of sympathy out there. My O/H doesn't run, but had to deal with similar issues in order to carry on with exercise classes etc at a similar stage. I seem to recall that the main advice she got was plenty of pelvic floor exercises!
In terms of dehydration, provided it's not a hot day, you may find you don't need to hydrate as much as you'd think. For the last few years, much has been made of the benefits of hydration during exercise -and I do believe it is necesssary for the longer distances and on particularly hot days. But for many years, people were regularly running half marathons without any water or other drinks. I did quite a few like that myself as a youngster. I've also just done a 75 minute training run and just took a sip of drink from a bottle I had with me. BUT, I reiterate, on hot days, hydration is MUCH more important! The main thing when taking on fluids, as you're probably aware, is to take on small amounts at a time.
Finally, really don't worry about it. I read the biography of one particular olympian, who was having some difficulty because of having taken on too much water on a particularly hot marathon day, at the olympics. Apparently, he was inwardly debating whether or not to stop when, as he put it, the problem resolved itself. So you'd be in good company anyway!
Good luck with Bham HM