Just based on your 800/1600 times you should be running much faster than 19:30. Unless you are exceptionally large, like over 6ft3 185lbs, your 5k time should still be under 18 regardless of your specific talents. Is it possible that you really matured or trained in the winter for the first time between xc and track last year? Are you running all your mileage on the track or roads? It is possible if you are bigger that the loss of bounce back when running on uneven grass/trails could be really reducing your stride efficiency (particularly if you are a heel-striker). What's your 2 mile pr? I gotta believe you could run under 11 pretty easily on the track. Keeping that in mind if you can be under 11 at the two mile mark you can definitely close last mile in around 6:15 putting you under 18. You probably need to do more easy running on grass/trails to build up your base and get your stride more adjusted to xc running. As an experiment at your next 5k try to go out really hard and hold on with the leaders until halfway and then just survive by latching on to each guy that passes you for at least 20-30 seconds. Hard to believe you wont at least get under 19 doing that.
If by pre workout you mean race warm-up then that depends, but I always found a cup of coffee/tea, about 10-15 minutes easy running followed by light stretching, and 6-8 100m build ups was a good starting point.