Or: On, Done, and Off in 45 minutes.
During the 12 years I wrote for Runner's World magazine, my Go-To Guy for any training feature was one of this country's premier exercise scientists. PhD. Head of the human performance lab at a major Big 10 university. Big name in the field, tons of street cred. He remains anonymous here because he now dispenses his info on an online subscriber-only basis. But I mined his expertise broad and deep while researching the many How To Get Faster pieces I wrote for the mag. And what he says, works.
One point he made over and over, both in phone conversations and email responses, was this (direct quote): "Intensity is the Most Potent Producer of Fitness". Specifically, short bits of harder-to-WAY-harder effort than even your normal hammer-ish tempo on selected sections of club rides will eventually make you stronger than longer chunks of, say, moderately hard riding. Think: Shorter, harder. WAY harder. Just not for very long. It's like marathon runners doing 400-meter repeats at a tempo a lot faster than their marathon pace.
Judiciously done, of course. Say, one of these suffer-fests every 7 to 10 days max over the winter.
So, what exactly are we talking about here?
Well, based on a bunch of double blind studies his lab did, one of the best, most efficient stamina-building sessions consisted of 2.5 minute repeats at hard to most-unpleasantly-hard to okay-that's-freaking-ENOUGH efforts followed by 1.5 minute easy-spin recoveries. Seems that 2 1/2 minutes is close to optimal: Long enough to let you really amp it up and hover just barely beneath the Red Zone, but not so long that you can't sustain this kind of hyper-high output (about the time you're running out of gas, time is up). And the 1 1/2 minute recovery gives you just enough time (barely; believe me. Barely) to semi-clear your legs and get your HR back into the 120 bpm zone to permit another 2 1/2 minute blast. But not so long that you are able to fully recover.
No, this isn't any kind of Magic Bullet workout. There's no such thing. But, apparently, the 2.5-1.5 combo platter IS one that works better than many others for increasing stamina (the ability to go "fairly long" at speeds faster than you previously could with the same effort).
Yes, yes. I know. Winter is for "base building". Mainly. Mostly. But slogging every gray-skies mile at 70% HR turns legs, brain, and spirit into lead. Makes more sense to add some intensity, even in the Dec-Feb dead zone. Moreover, during this workout, you are almost always changing the effort, so time passes far, far faster. And you do it indoors: On the bike, done, and off (dripping and panting) in maybe 45 minutes. Tired, but immensely satisfied, knowing that you have just done something substantial. Something that is making you stronger.
Do this 3-4 times a month for the next 2-3 months and see if you don't come into spring a notch or two higher up the Velo food chain than you are right now.
So, here you go. I prefer to do this workout on a LifeCycle at the gym as you can specifically and instantly change the resistance. But you can also do it on your indoor trainer or rollers. Your call. Whichever, here's what you do:
1. 10 minute warmup: 5 minutes of steady pedaling starting at level 1, then, over each subsequent minute, increase it to 2, 3, 4, 5. Follow this with 5 
minutes where you work in, say, the level 5-7 range (whatever feels right for you) alternating 30 second high-spin bursts with 30 seconds of recovery cruising. Get your heart pumping, the blood flowing into your legs, just starting to break a sweat.
2. Meat of the workout: Pedaling in the 90 to 100 rpm range, do 2.5 minutes of hard effort, progressing to very hard, progressing to DAMN hard (not really all-out. But close) so that your legs are bitching big time and your heart thundering when the 2.5 minutes are, mercifully, up. Start out at a resistance level that feels demanding from the outset, hold this for 1 minute, then kick it up 1 level for another minute (getting really hard now). Finally, add 1 more level of resistance for the last 30 seconds. While you never want to feel that the last 5 or 10 seconds is the absolute most you can do (again, NOT all-out; rather very, very hard. Just on the cusp of going anaerobic), you want to finish the 2.5 minutes feeling pretty pooped.
Example: 1 minute @ level 7, 1 minute @ level 8, 30 seconds @ level 9. Or 6-7-8. Or 8-9-10. This is the part you have to figure out. Use your first workout to experiment. Play around a little. It doesn't take long to come up with your own personal template.
3. After each 2.5 "on", punch in level 2 or 3 and spin easily at 90 rpm for 90 seconds. Then hit it hard again for 2.5 minutes.
4. How many of these little Four Minute Maulings do you do? Shoot for 4 x 2/5/1.5. If you're fried after that, spin easily for 2 or 3 minutes, then do a second set of 2 to 4. Your eventual goal is Eight Straight. If you do this workout right, that's plenty. Your goal is getting stronger, not to be admitted to the ICU.
5. Finish off with 5 to 8 minutes of easy pedaling for a cool-down and leg-flusher.
Remember: Once a week. No more. The rest of the time, just ride your bike or do longer, less intense indoor sessions.