I spent $400 on a Tacx flow trainer that provides me with a measurement system for cycling during the winter. I base 3 days a week on the Tacx and record every workout and either improve in watts, less recovery or more duration or I just get off the bike. When I start racing in March, I have the strength to ride hard and have the fitness to start riding 3-4 hour rides (that's all I need for the racing I do) right out of the box.
For the next 4 weeks, you might want to try what I'm doing and see how you like, maybe ditch one of the days if it's too much--
Every other day on the trainer:
Day 1 1 min on 1 min off, high resistence, 50 rpm's. do 10, add a couple per week. Rest a minute when done, do power tempo at 80 rpm's med resistence. Start 25 mins, add 10 mins per week. Warmup 20 mins, cd, whatever. Use a fan, HR should not be all that high.
Day 2 4 x 10 mins, 90 rpm's HR should be below AT. % mins recovery week 1, decrease recovery each week down to a minute.
Day 3 Tempo--below your AT, start 40 mins, go up 10 mins a week.
Do for 4 weeks, then take an easy week. If you have the facilities to do a test (i.e. borrow a powertap, end of the easy week is a good test time)
Go run and swim a lot on other days, if it's not blizzarding go ride 2+ hours outside on one of the weekend days.