Compared to St. Helena, your pace will be cut in half, on average, during the Whitney hike. Altitude is a big factor, but also the accumulation of continuous uphill hiking. At altitude you HR is higher. This in turn causes you to burn out faster than you otherwise would, making it necessary to slow down as you get higher.

My uphill (moving average) speed from the Portal to about Outpost camp is about 25 min/mile (when you factor in rest stops, which for me are about 10 minutes every hour, it's about 35 minutes/mile).

During the latter stages my pace declines to about 40 min/mile (50 min when breaks are added in). So the whole hike takes me about 14-15 hours or so (probably too slow for you, but normal in my age group).