Your line "I like to be with my partner" says it all. You've raised kids and are ready for a mature relationship in which each of you have pleasures and take pleasure in each other's company.
Not a bad life. I suspect there are other men who would love a wife who wishes they had a passion for golf.
Your fiance seems to have other objectives.
I don't mean to be rude but...Why does he want to marry you now if he's planning to be away 4-5 days per week for a year?
* Does he want someone to take care of him if he is severely injured?
* Is he blowing his retirement savings on the racing game and counting on you to support him?
* Is he so self-centered that it hasn't occurred to him that it doesn't make sense?
Without destroying his fantasy or giving up your desires, can you imagine a life that makes sense for both of you? If so, is he willing to work with you to make it real? If not, say "I don't":
"I don't want to complicate my life with someone with whom I am unlikely to be happy."
"I don't want to expose my financial security to the chances of the race circuit."
"I don't want to be married to someone whose passion leads him away from me."
Free advice means little unless one knows something about the giver:
I've been married for almost 20 years. I love and respect my husband, but we have a complicated relationship. I don't know if we'll be together for the next 20 years, but we have a center of gravity. He and I put issues in context -- we work together to make things work for all of us -- he, me and the kids. Not "Father Knows Best" but something that works for us, for now. The good news is that we all want to keep it working. The bad news is that it isn't easy.