one way in my mind now... Use HPWM on your led emitter, place the IR receiver on RA.4/T0CKI. Use Timer0 to count, Timer1 or Pause to do your sample time. The read the TMR0 register.

Another wich i don't really like but i saw many times... Led emitter directly to Battery via resistor, IR detector to RA.4/T0CKI. Jump in a interupt routine on a TMR0 overflow. Easy neat... may kill the IR Led soon.. battery too.

Do a search for T0CKI, you should find some tips to begin.