You are not the first to do this and will not be the last.
If it was me and I realised during the day, I would leave the Lantus until the usual time.
Until then, I would test and correct at least every two hours (but be aware of bolus stacking when you calculate your correction dose).
It is likely your high BG will make you feel drowsy so if you can correct, you may start feeling a little more alert ... and less likely to forget your Lantus tonight.