I also was told in a recent first aid class that without very prompt EMS and hospital ER followup and the presence of one of those automatic external defibrillator devices, that CPR is only a stopgap and not very successful in case of heart attack--something like a 5% success rate. The AED is not exactly something any backpacker would want to buy or carry, although it certainly should be in every public building!
On the other hand, CPR can be a true lifesaver in cases of drowning or lightning strike--both accidents that could easily happen to any hiker of any age. In other words, CPR is still very definitely a useful skill to know.
As for me, don't bother--take good care of my dog and let me go in peace!
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey