Taking a stab at the bike directions from 4th and Locust to 34th and Spruce clearly follows the bike lanes on Spruce 22 and Walnut St and takes about 20 minutes.
The second route I tried from my neighborhood in NJ to the BCGP office. It followed River Road in Burlington County and diverted to right side parallel streets when the opportunity to do so arose. Once in Philadelphia the route followed bike lanes but took me down Broad St around City Hall. Travel times may be seen as conservative but realistic when you factor in rest stops or traffic lights or the fact that not everyone is riding on a $1000 road bike. The overall speed appears to be somewhere around 8-10 mph.
The second trip shows the difficulty of bike route planning, different comfort with traffic tolerances. Google is assuming that people will want to avoid arterial traffic whenever possible. Not everyone will be wowed by this but I think its important to realize that this is a beta test and directions will have to be tweaked with local knowledge, which Google Maps has made easy by injecting a problem link into the directions page.