1 MHz clock speeds are not that uncommon in microcontroller pcb design. As always, use max trace width/spacing available, and shortest run length possible. If the opto is rated for 10Mb/sec, then it should be fine. Keep spacing to ground planes at a maximum, 25+ mils should be alright. Also, try to keep a plane on the opposite side from the runs, and split it under the opto, keeping the input ground plane away from the output ground plane and separate. Do not run input lines over the output plane and vice versa. The data sheet for the ade7758 should tell you if you need line terminations or not. I would think not, but I guess it depends on your layout. I've done motherboards up to and including 100MHz.