As they give out bottles you should be fine to use theirs as you can carry the bottle for a few k and take on all you need. Smaller runs that give cups are a pain and I prefer to carry my own. However as book said if you want it then do it.
Start to analyse how much water you are consuming in training. I am always sceptical on whether runners really need the volume provided in a camelbak (depending on distance of course).
I think the great runs give out powerade at half way which I advise (if you are going to use it) then train with it first. It did not agree with me at all.
Last piece of hydration advice form me is to have some flat cola in your kit for after. You can stick your power drinks flat cola is the only way to go just dont guzzle it.
I find them useful. I am busy at work and with kids so I find it hard to keep track of how many weeks I have before an event so like to map out the increase in mileage and pace so I hit the right levels at the time of the event. Getting the right mix of hills, intervals & tempo runs is always tricky as I sometimes cannot remember what I did last week so the plan helps me there also.
Sure you could do it roughly in your head but I just find it easier to note it down a few months in advance and can then check the plan everytime I leave for a run. If I miss one or need to move things around I am not bothered I don't get stressed about it I just find it useful.
Another great two castles thanks to all for organising. Heat gave an extra edge to an already challenging course. Agree with the OP I saw quite a few people struggling and getting treatment I hope all are ok.