You'll meet plenty of other people on this popular trail. Your dog is required to be on leash, and you'll want to hold it firmly. This isn't a great trail for dogs who are too shy, or those who know they can catch that bicycle if you just give them the chance! It is a bikeway, and many of the people you'll encounter use it as such. It's also something of a commuter trail; because it crosses city streets, locals use it to get around. You can avoid most of the crowds if you don't walk it at rush hour, but there's no guarantee that you'll have it to yourself.
This hike is a great beginner hike. It's long enough (especially if you go the full 5.3 miles out and back!) to be a challenge. It's most flat, however. It used to be a rail line, and though trains can cross mountains, flatter is better. Since then, the trail has been paved over. It's black asphalt all the time, making for an easy walking surface for humans. Keep an eye on the temperature, though. Black tops can heat up on bright days, so take care to walk your dog in the grass if the road's too hot. Off the path is where you'll see the wildflowers, too--provided you're there in season!
Sir Frances Drake Bikeway is furrific fun!