Walking with others is a super idea, I'd be careful about how I worded a sign though. Ideally, only good, kind people who want to start walking would read it, but there's always the chance that someone who isn't good and kind would read the sign and start keeping an eye out for the "short chubby girl making laps around the place in the late afternoon early evening." Personally, I'd feel much safer leaving a sign with my first name and phone number than a sign that told when I was going to be walking alone.
Also, I would hunt down a friend, family member, neighbor, etc. to go along with me the first few times I walked with someone I met through a sign like that. Even if they don't want to commit to regular walks with you, you should be able to find someone who would be willing to join you for safety's sake on a walk or two.
I live in what I consider a safe, rural neighborhood - we're not afraid to leave the house unlocked or the keys in our vehicles - but I watch the evening news. I don't live in fear, but it just makes sense to be careful.
Good luck to you, I hope you're able to find some great new friends and walking partners.