Hi there and thanks for posting this question.
I guess that if you are used to the activity you would be fine although i am concern on the length of time that you will engage in that physical activity i and I am going to tell you why. Whenever we push our bodies to the extreme and 6 hours looks pretty challenging you may increase catabolic hormones (destroying muscle, muscle protein breakdown) contributing to add some hormonal imbalance so if you are pregnant I would be careful. If you decide to do it at medium or low intensity it would be much better.
In any case feel free to post in the pregnancy forum at medhelp and I am sorry for the delay in this response and if you did it and everything went fine congratulations.....