Well, I don't really want to be the one to send you into a panic spin, but that's exactly the risk you're taking: gambling $150K on getting that job.
But I don't think it's time to stress about it yet. Consider a few things:
1. Unless you attend a poor school, your chances of passing the bar and finding a job within 9 months are pretty good (80+%).
2. Once you have that job, while it's a bit of a pain in the ass, it's easy to stay in the US indefinitely as a Canadian citizen.
3. You actually CAN practice in Canada with a US law degree. Some firms in bigger cities look for JDs to cover certain practice areas. You can also practice in legal fields that do not require a call to the bar with just a JD. Furthermore, an extra year of studies at most Canadian legal institutions can give you the chance to be called to the bar in Canada, so it's not like the education was useless to you. And it will give you an area of expertise that many others lack, which is appealing to most employers in the field.
4. And, after five years or so, it gets much easier to transfer between the two countries, and you should already have paid off a large amount of your loans, so that won't be weighing on your mind anymore.
5. Even if all else fails, it's just debt. Yes, it's scary, and yes, if you don't take it seriously, it can debilitate you for life, but if you're reasonable and intelligent about it, 9 times out of 10 you should be able to find your way out of it. Okay, it'll feel like the JD was a waste of time, but you never know when it's going to come back to be in your favor. And that's not something you can worry about now, anyway. You need to LIVE your life.
The risk is great, but it's not dangerous. If your reasons for coming to the US to practice law are good, then I believe it's a risk worth taking. So don't panic.