Here's my two cents. I think you most likely "could" do it. But I also think you might be happier waiting to do the 62 in October.

I basically agree with your idea to get in more miles and see how you feel right before the event. There's a lot to be said for "base miles". The more you get in before a long event, the easier that event will be. You might be able to do the 42, but it might be unpleasant (or it might not be). Maybe you don't care about unpleasant, though, and rather love the idea of accomplishing it one way or the other.

I had done a century on a tandem and almost a year later was training to do my first century on my own bike. As I amped up my miles, I was having a hard time. I did a 60-ish mile ride a few weeks before and was pretty miserable. I decided to ditch the century idea and do the metric (about 62). I was SO much happier doing that ride, which I was more physically prepared for. And using that ride as a training ride I ended up doing a full century three weeks later.

As a side thought, after that miserable 60 miler, I also had my bike professionally fit, which made a huge difference in my comfort and riding ability.

Anyway, it's good that you have flexibility and options. When it's closer to the event you can better determine if you are ready. If not, you can keep getting in miles and probably do the Metric in October just fine.