use a trainer? and adjust the resistance to max. (or adjust to something much harder than you normally work). Ride on the trainer for two to three hours as if you were climbing long hard mtn.

Before I quit riding 6/7 years ago* I used to adjust the resistance and train for several hours. 15 minutes of slow warm up. then increased my output in one to two minute interval for next 15 minutes then my real work out for about an hour and half. Follow this by cool down (important). keep water bottles in cage so you don't dehydrate. I used to listen to music to help pass the time and also used timers for the interval workout.

Sorry to hear about the weather though. And good luck with your ride. which ride??

smilingcat

* I'm back on... riding recreationally... well sort of.