Seek out your local Sierra Club hikes if there is a group nearby.
I am not a member but I join them weekly on their local hikes. From my local city park they meet 3 weekday evenings and 2 weekend mornings every week, year round.
Groups are divided up into different skill/difficulty levels and the hikes generally last about 2 hours. It's a great aerobic activity and a good way to meet new people.
Here are a couple of photos from last Thursday's evening hike. We had about 15 people in this advanced group, but some easier groups can be as large as 30 or 40. people.