Here is a high level view of one solution:
Connect the LAN side of the routers and the outside of the ASA in a single VLAN. You will need an additional switch if your routers do not have multiple Ethernet ports.
Configure HSRP on your routers, and make the VIP address the default gateway of the ASAs. The router's static route to the inside is the ASA primary address.
Configure HSRP to track the interface of each WAN link. So if the WAN link goes down, the HSRP priority is reduced, causing the other router to become the active peer. In this way, only the router with an "up" WAN link will be the HSRP active router.
If you want to get fancy, you can set up IP SLA to ping the ISP to verify reachability, not just interface status, and let HSRP track that. Or, you can run BGP with your provider (default route only) to verify reachability.
Let me know which way you want to go, and I can come up with some sample configs if needed.