That's not necessarily true. The length of sessions are based on level (Fit, Trained, Athlete) and number of sessions per week.seat5 wrote: it seems like the IP pretty much neglects longer distances at fast paces.
To find more demanding workouts, you could either
1) stay at your current level (Trained, I presume) and look at the program for 5 days a week. I think those sessions will be slightly tougher. You could simply drop one of the five and do four sessions a week.
2) increase your level (to Athlete) and look at the 6-days/week program and pick the 4 sessions that give you the right balance between longer and faster workouts. You will see that the "Athlete's" plan is tougher.
3) You could simply add an interval a couple of days a week. If the plan calls for 2x15, you could do 3x15.
Of course, we have to be a bit careful improvising. The program is designed to train all the body's energy systems and to build cyclically. But I expect you can make the program more demanding w/out violating the underlying principles.
Tom