While exercise bikes usually provide an effective 'distance ridden' reading that can be entered in as a manual ride, many spin bikes produce no such measurement. 

For those on a spin bike that doesn't provide a reading, you can use  the following conversion chart as a guide for recording spin classes:

15 minutes4.65 miles
30 minutes9.3 miles
45 minutes13.95 miles
60 minutes18.6 miles