116 degree heat index?!?

I'm getting more and more worried that I won't be able to complete this century ride I signed up for. I've not had much time off lately. The time off I've had it's been raining/ popcorn storms, or like today: too hot to safely ride.
I've been cross training in the gym and doing spin and stationary training bike. But those are nowhere near the same thing. What am I going to do? I think I may be in over my head.