I have learned this after many years of riding... if I think I can do something, I can do it. Any physical activity is 90% mental. You have done a century - 45 miles is nothing! So you get sore and tired, so what. That's the "fun" of doing an special event ride. I did 70 miles last weekend after not doing more than 30 - 40 one day a week. I was tired, it wasn't easy (especially since that included 4500' of climbing!) but I put my mind to it and just got it done. Sounds like a cool ride - go have fun!