Looking at the Data sheet of the chip 18F2455 and especially in the A/D Acquisition Requirements, it seems that you need a 2.45usec for every sample. Of course you need also a little time to transfer the AD value to an array and start AD again. Say for the sake of the discussion the the total time is 3usec.

This leaves you with only 20 samples in the 60usec period.

Is that OK with you?

Ioannis