How long are these workouts? Unless you are in great shape already, you probably cannot sustain 90% for very long. You might be able to sustain 80% for a bit over an hour, but then you'd probably need a day of recovery.
They will, however, increase your VO2max such that you can probably sustain a lower HR for a longer period of time at a higher speed so those long 3-4 hour workouts, also known as LSD workouts, become possible.