Same thing happened to me during my first IM in Penticton in 1999. Everyone told me it was due to dehydration. I had a bit of stomach flu leading up to the race and was probably not as hydrated as I should have been prior to the start and even though I drank a lot during the race I was reduced to a shuffle by 10k into the run with tingly fingers and super heavy arms. I just slowed down (like I had any choice?) and made sure I drank a ton of gatorade at all the remaining aid stations.