K.P.
I lead hikes for the 3 day Breast Cancer walks. 1 good flat one is at the end of Grand in san Leandro. It's 5 miles to the San Mateo Bridge and 5 back to the parking lot. Or you can start at the San Leandro Marnia to make it a little longer. Another is around Lake Chabot in Castro Valley ,It's 12 miles around. It's alot of hills though.The Iron Horse trail in San Ramon is flat and you can go all the way to Alamo. Good luck and have fun! Don't forget to drink lots of water....K.