The boardwalk between Mission Beach and Pacific Beach is around 3 miles each way. If you walk from Belmont Park to Pacific Grove, it's a touch shorter at 2 miles each way.
At one end is Belmont Park http://www.belmontpark.com/ with a large wooden roller coaster, amusement park games, and the Wave House http://www.belmontpark.com/restaurants/wavehouse/ where you can have a drink and watch surfers try their skill.
At the other end is Pacific Beach, where you will find hotels, restaurants, bars, and beach shops.
If you're looking to hang out the San Diegan natives, the boardwalk is the place to go and has the best people watching.
Tip: There's a lot of free parking on the Mission Beach end by Belmont Park.