Well, I think intervals is anything at a pace that you can't sustain continuously. It's a different pace for everyone. I know all about comparing myself against what I used to be able to do ... and I also know that that is really un-helpful for improving from where I am now. (Not that I've been able to get past it on the piano the way I have on the bici... )

That alone might be a reason for you to try shorter, more intense intervals with shorter rest periods. You could run at an intensity that's at least somewhat familiar to you from the past, for a duration that's possible for you in the present.