I have made this walk several times and it is no more than 10 minutes from Building 5 FQ to the River Roost at Riverside. We go to see Bob Jackson every visit, or to eat at Boatwrights. Of course my 6 year old sometimes gets diverted with a duck or bunny. If you are in reasonable health, it is very enjoyable. Definitely faster than the internal bus unless you just get lucky and it is at the stop waiting for you.
Artemus