I'd do it, particularly if you're feeling fit. Although, i do know that my fit and running fit are quite different. But i do find once you get past that first hurdle, and get your legs rolling over things get easier. HAving said that. i've only ever run 12kms!!! So i don't know what the next pain barrier feels like. But no one will be there with a cattle prod, (or will they?) so you could always pull out if it was too much.