Forget the Bike Coalition for organized rides. They are an advocacy organization.
For group rides, check out
www.phillybikeclub.org
I think you can rent bikes at Breakaway on the 1900 block of Chestnut in center city. You can also rent from Fairmount Bicycles, which is near the Art Museum, which is near where the trail system starts. Look them up on the web.
On weekends, MLK Dr., which starts at the Art Museum, is closed to motor vehicle traffic from 6 a.m. to 5 p.m. (The first 1.5 miles opens to cars at noon, but there is a path you can take.) It's nice and shady and beats the crowded path on the other side of the river.
While theft varies by area, the safe rule is don't let your bike out of sight unless you lock it up with a good lock.
If you are not used to urban neighborhood riding, you might feel thretened riding from the airport into center city. Take the train. You can bring your bike with you on the weekends and during off-peek hours on weekdays. If the shops make you return the bike at the end of each day, there will be no need to worry about any of that.
The SRT takes you to one side of Valley Forge Park. If you want to see the side where Wasington camped, you have to cross the river via a narrow walkway along side a bridge. There is a sign just before you get to the Betzwood picnic area direvting you to the walkway, which will be on your left. From the Aret Museum to Betzwood is about 23 miles one way.