Trek1500 - do you know which bone was fractured, (eg radius, ulna, scaphoid or something else)?

Is you thumb mobility limited by the cast?

I would try riding the trainer before going out anywhere. A lot of whether or not you can ride depends on it you can get into a good position to ride. I also agree with Pika, it's good to be able to elevate your wrist periodically. You could do that on the trainer. You also don't want to put yourself at further risk by being unsafe because you can't brake properly.

Finally, if it happens to be your scaphoid, don't mess with it. They are a PITA when it comes to healing.

I fractured my scaphoid 2 years ago and it was hard for me to get back on the bike. I was casted for 8 weeks and I was too painful to ride for about 2 weeks after the cast came off. Then it slowly came back. I was able to ride normal amounts about 6 weeks later.