anyone playing a brass instrument? a week to get fit...

[MousyMouse]

have agreed to playing in midnight mass on christmas eve.
have not played since last christmas...
usually I start a month before to get stamina (mass is 1.5 hours, pieces are long) and height.
due to being ill since beginning of november (cold and very persistent cough) I only was able to start yesterday.
tips and tricks?
playing in an ensemble of 5, so nowhere to hide...

[MousyMouse]

just did an hour of low flexibilities and my mouth muscles hurt...