It helped me to know, even though I'd convinced myself I'd got GBS before I went in to the neurologist. It was therefore good/bad to hear the news, but overall good to have something to focus on.
So in answer to the question - I think the right response is probably "pisser, but at least you know" or something similar a bit more delicately put.
To all you runners out there, as a previous keen runner I TOTALLY get what you're experiencing. Quite a number of years ago now I came within 12 secs of my life goal of 1hr for a 10 mile road race, but then my achilles tendons gave in bigtime. More recently when I got the MS symptoms it hammered back my current feeble jogging efforts equally badly. Now as a result I get just as much pleasure from a walk as I used to from a run, as I'm just so thankful to have the mobility I've got. So what I'm saying is - it could just as well have been a running injury that stopped your running career as MS. I do hope you get back as much of your previous ability as you can though, and that you can still enjoy it just as much.