If you've got 28 minutes, you have time to make your tush burn!
A combination of high intensity "sprints" and reverse pedaling, this workout will give you a big bang for your time buck. Your butt will thank you later.
A few notes: go as fast as you can on the 30 second sprints, AND when going in reverse. Press through your heels to make sure you're engaging your glutes.
Don't feel it in your tush? Lean back so your weight is on your heels. (Your hands should not be holding on to anything - that is cheating!!)