In southern California, the freeway system is a monster during rush hour peak times.
Example: how far is Disneyland from LAX?
Typical answer is: right now it's at least a couple of hours, but after dinner probably 45 mins.
Back on the hiking trails, there is a popular book author, that posts trip reports in his hyper mileage speed, but his average readers are always getting stuck on the trail in the dark. Elevation miles are more demanding than flat trail miles. While he often brags that a summit only took 3 hrs, his typical readers get screwed with misleading info.
Also, for myself, in the early morning, I spend extra time getting ready, and taking sunrise photos and slow moving, whereas in the afternoon haze, I just want to hurry up and get done for the day. So my uphill in the morning is about 1.5 mph, and my downhill in the evening is about 3.5 mph.
I would suggest decyphering a topo map to get an idea of your personal speed for altitude gain, mileage, terrain difficulty such as boulder climbing or weather (ice vs dry dirt), correct gear for terrain (ice axe and crampons vs trail runners), pack weight.
This looks like an algebra challenge for forecasting duration under the above conditions.