I think flirt poles, spring poles, tug, and any water exercise help a lot with conditioning.
With weight pull, well I just feel like GSDs are fragile for so long lol [referring to their hips here and having to wait a year plus to do any high impact sports] ! My last dog was a pit mix. He didn't love weight pull, but we were going to try it. I bought him a harness and everything...and then never got to use it.