I'd say you should try on the 8s to be sure they're comfortable for you. I was someone who'd used Mizuno wave inspire 4s, 5s and 6s but hated the 7s. For the 8s they've gone back to more like the fit of the 6s. This has suited me, but you may find they feel different. If you don;t want to try them in the shops, for marathon training I'd possibly reccomend getting another pair of 7s as you have got on well with them and this might be cheaper to do online than getting 8s. Hope this message makes sense - too many numbers in here!
Edited: 16/07/2012 at 11:45