Hi
@Deano79 - although I'm currently training for a (mere!) 10k I don't have a long history of running, and although I have run 8-10k a couple of times, my more frequent runs are 5-7km. I have worked out what works for me.
I'm on a pump, and the time of day that I run is either 9/9.30am or 7/7.30pm. If it's the former, I'll've had 2 boiled eggs with my normal breakfast bolus insulin at my normal breakfast time, and before starting my run I will set my pump to 50% of my normal basal rate for the duration of the run. I find that running makes my blood sugar rise for a couple of hours afterwards, so as soon as I've finished my run I will set my pump to give me 130-150% (depending on how hard I've run) for two hours.
For my next meal I tend to have only half of my normal mealtime bolus insulin, otherwise I drop. If I find I'm dropping more (which can happen for me 6-8 hours post-run) I lower my basal rate again for a couple of hours.
It's clearly not an exact science, and I find this frustrating - but post-run I keep checking my Libre to see where I'm headed. I couldn't be without it - although as
@Jeff_uk says the sensors don't co-operate in the cold, so I find scanning during a run is - at the moment at least - fairly pointless!
I'd thoroughly recommend the Animas Sports Weekend to any type 1 who's interested in exercise - you don't need to be an Animas customer, just a type 1 - I did this in May last year and it was really, really helpful. If you drop them an e-mail they'll let you know when registration opens for their next one (and places sell out fast!).
https://www.animascorp.co.uk/SportsWeekend
Hope this helps!