I keep a close eye on him. He is the biggest pampered pet you could ever meet, as evidenced by his new pumpkin sweater.
It was only down and up our hill. Then off the leash to tucker himself out at his own pace. Believe me, when he is tired he stops and watches me ride back and forth while he sits in the grass. He loves the bike though, and goes crazy when I start it up.
I am aware that dogs can hurt themselves to keep up with their owners when distances get long or runs get too intense. I would not let that happen.
I started him with bicycles as well. It is great exercise for the dog. It takes time to train them not to go in front of the wheel, but it can be done if you are consistent with the training.
As for me, I wear full gear when I ride anywhere, but I was only going running speed, on our hill, so I figured it would be fine.