Here are some helpful resources
This Code Gallery project contains the code samples to help .NET developers better understand and evaluate Windows Workflow Foundation (WF) in the Microsoft .NET Framework 4. The code accompanies a collection of documents that the WF product team is providing to help .NET developers currently using WF in the .NET Framework 3.0 and 3.5 to evaluate .NET Framework 4. The documents map concepts between the existing WF model (defined as the types in the System.Workflow.* namespaces, or WF3) and the new WF model (defined as the types in the System.Activities.* namespaces, or WF4).
The WF Migration Kit helps users migrate WF3 (System.Workflow) artifacts to WF4 (System.Activities). The high level goal is to migrate workflow definitions and declarative conditions, but not code (such as workflow code-beside methods). Some WF3 workflows will be fully migratable to WF4, while others will be partially migratable and will require manual editing to complete the migration to WF4.
The WF Migration Kit provides an API as well as a command line executable tool, which is a wrapper around the API. Migrators are included for some, but not all, of the WF3 out-of-box activities. The WF Migration Kit offers an extensibility point so that custom activity migrators can be developed by third parties.
WF3 did not have any kind of host. WF4 now has a great host provided with Windows Server AppFabric. You really should look into this and use these two together.