You need to increase your muscle strength and stamina. Working out in general will help, you didn't say anything about your workout program, or if you have one, so any advice is going to be a shot in the dark. I'd suggest lifting after your cardio work, so that you build the strength when your muscles are tired, teaching them to perform when tired. You won't gain size that way, but it will help with late-race strength. To work your forearms, simply use forearm curls. Lay your forearms on a bench, or your knee, and curl the weight up at the wrist, don't use any upper arm. Make sure you're working both directions( with your hand facing up and down), and use hammer curls during your bicep workout. If you don't get in the gym you can use newspaper to help build strenght. After a hard ride, grap the paper, and use full sheets, holding them by the corner, use one hand at a time to crumple them up. Do several sheets per hand, as many as you can do.