Training & Nutrition - Intervals on a short commute?

I´ve been pondering on how to make my daily winter commute less cold and monotonous...
So I thought why not make it a workout and try to improve my strenght and raise the tempo a bit.
However my commute is not more than about 15 miles both ways so I thought intervaltraining?

I know I could just make it longer by taking a detour but currently I also go to the gym, run, and my job as mailman has me running up and down stairs a good portion of the day and since I also would prefer that life contains other things than exercise, making the commute longer is not really an option right now.
Call me lazy:o.
Note that It´s not an ambition of mine to become an elite cyclist.
I just want to be able to ride a bit faster.

Anyway, my question is if it´s possible to do some decent intervals and strenght training on the bike with only 15 miles a day?
And if anyone has a good idea on how a week could look like I´m all ears.
Any links or suggested reading would also be welcome.

Thanks!

15 miles is plenty of time for some fun.

google "long steady distance interval training"
http://turbulencetraining.blogspot.com/
google "tabata intervals"
google "high intensity training"

Thanks alot for the tips!
Found alot of good reading.
One question though. It seems like the majority of the training i found focuses on fat loss.
This isn´t really my goal as I want to increase my strenght and go faster basically..
But can i still go for the same kind of training?
Another thing I´m a bit curious about is how to plan the week so to speak.
I want to increase my strenght so I should probably incorporate some intervals with a heavy gear.
But still have some days where I focus on keeping a high cadence?
Am I right so far?
Again looking for some tips or links on how to make a good combination of the two.
Perhaps some sample workouts or week scheduals to get an idea on how it´s supposed look?
Questions, questions, I know.
But I´m a bit of a greenish amateur at this.
Sorry =)

I usually do one workout a week, 50-60rpm in a high gear for about 20min. Another day I do spinups (highest cadence you can do without bouncing out of the saddle) and one leg isolation drills.

You should check out Friel's Cycling bible.