Good luck on it :) Ok, firstly I can't run, so can't advise on that and won't! But coming from a biking stand point, and being relatively new to a pump (less than a year) I can throw a few things in.
When I got the pump I did have a fair bit of retraining on dose handling though. It's probably going to be harder for you as I doubt you will be carrying a large stock pile of high carb drinks as it's possible to do on a bike. But looking at my training readings off a combined data from a CGM sensor and testing (CGM can't totally be trusts as it's glucose in the fat layer as opposed to the blood tester, but is great to show trends), and discussions with my dieticians it was possible to work out a carb dose for so much riding/exercise. In my case it turned out an insane or so I originally thought 10g per 15mins approx (to carry enough liquid with that kind of dose I ended up using things far stronger than normal off the shelf energy drinks, the mix your own things to get this kind of potency, although I believe fitness level also plays a part in how much will be needed). But that was after the first hour or so. The first hour or two depending on ride out, I was coping quiet nicely off a nice stack up on porridge or some longer burning food. After that though it was pretty much drip feeding the carb drinks till a food stop, which even then only provided energy bars/oaty Cliff Bar things which gave a steady supply for the rest of the ride.
Blood wise, depending on delays at the start I find the beginning of events the hardest with BG raising to the 9mmol's (say 160 ish) or even scraping a 10 (180). I found short dosing (say 25% of normal dose for the food) before the event was good. But, the best bit is where the pump really comes in with the basal dose during the event. Unlike my old days with injection, having total control over the basal is fantastic and being able to set that to 10-20% of normal usage while riding gives just the tiny dose to keep everything working nicely. And the food working as intended, but giving the stability to keep on ploughing in the miles.
I know my pump which is a Medtronic Paradigm isn't meant to be properly waterproof, but find a zip lock bag works wonder for that and stashing it down your shirt even in serious monsoon weather that's bouncing it keeps things going. On the cannula side a waterproof patch over it works brilliantly also. So weather wise (for water anyhow so mud should be ok) can usually cope fairly nicely and cheaply without having to buy expensive cases and such from the manufacturers.
The biggest thing is getting a pump as soon as possible and banging in lots of practice which you are doing already in the gym, but also on possibly similar terrain to gather as much information as possible. Information is the key, as everyone of us is different on how we burn our energy reserves and what will need to be done to basal to keep things smooth.
I seem to be wittering on endlessly here :) So I'll shut up now. Good luck on the 5k :)