Anyway, if I walk slower one day and my heart doesn't go as fast, but I walk 90 minutes, that should count.
Says who? Apple set a standard for the Green exercise ring that is in accordance with WHO and other organizations who's research shows that 30 cumulative minutes of brisk exercise has a positive affect on your health.
If you walked 90min and got only 10min of Green ring activity (exercise) then either your Watch is defective or you failed to meet the criteria of the Green ring because 90min of slow walking with a slow heart rate is not the same as 30min of brisk walking with a faster heart rate. If it was then 100min of sitting on the couch or window shopping would be the same as 30min of actual brisk exercise. So no, little to no credit just because you did 90min.
To help insure you get credit from a brisk walk, make sure you let your arms swing naturally. Keeping your hands in your pockets reduces the accuracy.