A couple of ways -
1) Wakeup every 2 secs or so and count off secs, go back to sleep until you hit
the 1 hour point. Not exactly elegant.
2) Use the Low Power comparator, and a CAP and a R, to effect a long term delay/interrupt.
You would have to account for all leakages and R tolerance (R for charging cap) to control
the timeout. Use an open drain output to reset cap when going to sleep.
You could develop a simple cal routine to measure the time constant with A/D to improve
accuracy of RC approach.