Although you state that the coat of linseed oil wasn't excessively heavy, I suspect that it was too heavy.
Oils like linseed oil and tung oil need to be applied rather lightly, allowed to soak for a short time, and then completely wiped off. If too much oil is left on it gets soft, tacky, and doesn't harden properly. If you soften what's on there now you can wipe it off and begin again. It can probably be softened with another application of linseed oil or acetone or lacquer thinner.
Personally, I use gasket lacquer but polyurethane is easily available at the local stores and it works great. Spar varnish is another good one.
Guy