Looking at the website for the challenge (
http://www.thenutschallenge.co.uk/faq/), they say you can leave things at the "refuel station". Maybe you could take your pump off, do the 7km first lap, test at the "refuel station" (and top-up on either energy or insulin) then do the second 7km lap? That would seem like a reasonable approach to me.
I find it much easier to manage BGs during exercise on the pump than I did on MDI. It takes a bit of trial and error, but if you start off using a temporary basal rate lower than you need (e.g. -50%) then you can always adjust it up. I find most exercise only needs maybe -10% to -30% for me, but that just tends to be hiking or (relatively tame) cycling.