You have to get Epona when you reach the future. If you rush across the broken bridge in Gerudo Valley using a bunch of carrots, Epona will jump over it.
If you don't know how to get Epona here's how:
You have to have learned Epona's song from Malon as a kid, and when you reach the future you have to go to Lon Lon Ranch, pay to ride the horses, play Epona's song to make her come to you. Ride her around the pen until the time is up (jump over the center of the fences for 5 rupees each), then pay again, play the song, mount the horse, and talk to Ingo while sitting on Epona. He'll challenge you to a race. Win the first one and he challenges you to a second one, the prize being Epona. But once you win that, he traps you in the ranch. To get out, rush to one of the spiked fences and use a bunch of carrots to jump over. After that you can call Epona from anywhere in Hyrule Field, as well as in Lake Hylia and Gerudo Valley/Gerudo Fortress.
I don't know if you've gotten Biggoron's sword before but you need Epona to get it. Plus you need to have planted a magic bean in the cave entrance to Dodongo's Cavern so you can reach the top of Death Mountain in time for one of the last steps.
This is all in my walkthrough... it's better explained in there.
If you don't know how to get Epona here's how:
You have to have learned Epona's song from Malon as a kid, and when you reach the future you have to go to Lon Lon Ranch, pay to ride the horses, play Epona's song to make her come to you. Ride her around the pen until the time is up (jump over the center of the fences for 5 rupees each), then pay again, play the song, mount the horse, and talk to Ingo while sitting on Epona. He'll challenge you to a race. Win the first one and he challenges you to a second one, the prize being Epona. But once you win that, he traps you in the ranch. To get out, rush to one of the spiked fences and use a bunch of carrots to jump over. After that you can call Epona from anywhere in Hyrule Field, as well as in Lake Hylia and Gerudo Valley/Gerudo Fortress.
I don't know if you've gotten Biggoron's sword before but you need Epona to get it. Plus you need to have planted a magic bean in the cave entrance to Dodongo's Cavern so you can reach the top of Death Mountain in time for one of the last steps.
This is all in my walkthrough... it's better explained in there.