((((mmswm))))
I think there isn't, necessarily, an adequate way to express your thanks, and maybe the best thing you can do is accept that, not forget the obligation, and be there for your friends later if they need you. And keep in touch with them even though you're moving far away and have a lot going on, so they don't feel like it's "out of sight, out of mind."
I personally would find meaning in a heartfelt, specific note written to me by a friend in this situation, something I could keep and look at later. And that would cost basically nothing to make, but time.