Inappropriate sinus Tachcardia, anyone familiar?

My electrophysiologist said that I either have deconditioning that is causing a high heart rate after excercise, or I may have inappropriate sinus tachycardia.  I have had an ECG, several EKGs, a 24 hour heart monitor, and a stress test done for tests to diagnose me.  Other than a high heart rate after excercise, No one has seen anything otherwise wrong with me.  Blood tests did not show abnormal thyroid hormone levels or anemia, or dehydration.  I am a 32 year old Male.

I intend to pick up my level of excercise to rule out deconditioning, but I don't think that is the cause.  I don't get a lot of exercise, but it shouldn't be so little as to cause serious problems.

My resting heart rate is normally 72-74, although sometimes at night watching television it will be 60's.  Sometimes during the day my resting heart rate can be 80-86.  On my 24 hour holter monitor, my average heart rate was 75.

My only real problems tend to come from exercise.  My pulse often stays high after doing exercise.  Sometimes after moderate exercise my pulse can be 90-120 for a couple of hours after a workout.

I have a couple of questions for someone who is familiar with Inappropriate Sinus Tachycardia.

1.)  Does IST sound like it is my condition?
2.)  A symptom of IST is a high heart rate after very little exercise.  How little exercise is this, though?  I might be able to get my heart rate up to 130 after walking for 5 minutes (although I am overweight), does this qualify?  During my stress test, It took 9 minutes to get my heart up to 188.
3.)  Most importantly, is IST a relatively benign condition?  Is the only thing to fear from IST the symptoms you experience?  Is there any reason for someone with my symptoms to pursue treatment options?

Any feedback would be greatly appreciated.