Here's a Joe Friel workout that I'm doing.
10 Minute warmup in an easy gear and 80 - 90 rpm
10 Minutes 53 * 15(for me) you may want a 12 - 13t 50 rpm HR in Zone 3 - 4
5 Minute recovery in and easy gear and 80 - 90 rpm
Repeat 4 times, substituting the last 5 min recovery with 10 minutes of cool down 80 - 90 rpm
1hr and 15mins ---- Pretty good workout for a cold rainy or snowy day inside.