It probably depends greatly on what race you're training for and when it is.
Stopping in the middle of a tempo will do fairly little to detract from the neuromuscular benefits (strength, coordination, etc), but if you weren't a mid distance guy and the 20minute tempo were part of the metabolic work for a half marathon you're running a month from now, it's less than ideal to be stopping in the middle. And of course it's not likely that clear cut either way.
Either way, it happened and isn't a big deal. Hopefully you can figure out how to not let it happen again.
Good luck.
P.S. As far as 20 minutes being enough or not enough for an 800 guy, it's a pretty unanswerable question. Could be too much could be not enough, and depends on way more than duration.