Switch from endurance to HIIT with a series of hard sprints over a shorter ride. IMHO it is good to mix things up a bit rather than doing the same old routine for extended periods. Winter is a great time to work on increasing VO2 max with shorter but more intense training rides. Hill repeats or wind sprints are good places to start. I also like to add some extra cross training including more resistance training with free weights and/or machines.