Google Maps Transit – Customize Foot Speed for Directions

I often use transit directions to get around, but sometimes I'd rather run five minutes to catch up to a bus instead of waiting where I am for 15 minutes for the next bus.

Google Maps transit directions seem to assume a foot speed of about 10 minutes per km (16 minutes per mile).

Does anyone know how I would go about customizing this speed so that I can get to my destinations as fast as possible? I have some programming experience so I could use their APIs if necessary. I just don't know where to start.

The Austin Public Transit website allows you to specify slow, medium, or fast walking speed. I'd like to do something similar, but all over instead of just in Austin.

I've looked at the Google Directions API but it doesn't seem to have any sort of speed variable.

EDIT: Apparently Google uses variable speed, so a direct input of speed wouldn't be possible, but it would be nice if I could tell Google Maps transit directions to take their calculated walking time and cut it in half, for example.

Illustrative Example

It's 9:50am. I'm at the park when I realize that I need a cinnamon bun as soon as humanly possible. This craving is so strong that I am willing to run at a speed of 7 min/km to get to my tasty pastry. Is it quicker if I take advantage of public transit?

Option 1. On Foot Only

Google says "Walk 3.9 km, 48 min". It's easy to calculate that for me it's "Run 3.9 km, 27 min". In other words, I'll be enjoying sweet cinnamon goodness at 10:17.

Option 2. Transit Directions

If I follow the directions, I'll get off at a stop near the mall at 10:23. Again, assuming I'm really quick, it'll only take me 4 min from the stop to the mall, meaning I'll get to indulge at 10:27.

So I should run, right? Not necessarily. After some digging, it seems that I can get to the mall even earlier than 10:17.

Secret Option 3: Catch an Earlier Bus

According to the Transit schedule, Bus #43 comes every half hour. Google showed catching #43 at 10:27 as one of the options but it turns out I can probably make it in time for the 9:57 bus. With this bus, I'll get off at a stop near the mall at 10:12 and be there by 10:16.

Things get even more complicated when there's transfers involved.

Also, I'd like this to be automated because I get these sudden cravings often. No time for manual calculations and corrections!