Recording ReWired Tracks in Reaper

Seems like it should be pretty simple. I have Loaded Reason4 to Reaper via Rewire. I Have 4 channels of sound coming from Reason ,into Reaper: TRK.1 - Drums TRK.2 - Bass TRK.3 - Synth TRK.4 - Lead I press play in Reaper, and the 4 Reason tracks play through their respected reaper tracks. Everything seems fine. I have lots of VST's and things going on in Reaper and my CPU is getting taxed, so I want to 'import' or 'record' or 'bounce' those Reason tracks into Reaper. So I want to RECORD their outputs ONTO each of their Reaper Tracks. I hit record, and it doesnt record (records silence). So I think, well i will right click on the Reaper track and select "Render a stem tack and mute the original" I Do it, and it only record 2 seconds. Those 2 seconds of audio it recorded are perfect, but... the song is 5 minutes long. I have the loop set to the lenght of the entire song. So it seems this is possible, since it recorded 2 seconds just fine... So - how do you record ReWire tracks into Reaper? Thanks in advance. I have searched this forum, but havnt found an answer. Sorry if this has been asked a million times before, as i though it would have been.

Set the rewire track's recording mode to Record: output and start recording. Why the render failed is beyond me.

Render failed because your project has no length.

thanks for the tip on making a midi object the length of my song. I appreciate the feedback.