I live and sell homes in Martinez so I ought to be able to answer this one. As "the crow flies" it is north and east of Berkeley overland about 15 miles or less. Martinez is the county seat of Contra Costa County and sits on the San Joaquin river banks. If you were to drive from Berkeley you could take two common routes. You could drive towards Sacramento up Hwy 80 about 9 miles or so and then exit right onto State Hwy 4 east. Travel on Hwy 4 east another 8 miles and the first (and primary) Martinez exit is Alhambra Ave. Turn left and Alhambra Ave will take you all the way down to historic Downtown. Turn right onto Main St and immediately look to your left where you will find our office nestled between some of the most fantastic restaurants in town. About a 25 minute trip with no traffic.
The other route is to travel east on Hwy 24 thru the Caldecott Tunnel, through Orinda then Lafayette to Walnut Creek. This is about 9 miles. Hwy 24 then "tees off" into Hwy 680 north and south. Take 680 north for 6 miles thru Pleasant Hill and Concord and exit onto Why 4 west towards Martinez. Again travel on Hwy 4 to Alhambra Ave (about 3 miles) and now you will be in Martinez once again. Each route takes about the same amount of time although the second route is a few more miles.
And lastly, You could take the BART to North Concord. But that is still about 5 miles from the center of Martinez.