Ask HN: How secure would this password be, practically speaking?

So I currently have a rule for myself that I don't let any new passwords be shorter than my previous password. As you may guess, my passwords are getting quite long after a few years of having to change every N days. This got me thinking.

I know it's low entropy, but as a guess on how most modern brute force attacks work, I was wondering how secure something like this would be, both conceptually and practically:

1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaAaaaaaaaaaaaaaaaaaaaaa