Concepcion is a little more challenging. If you're running marathons you'll be fine. I hiked it a year ago, and had no problems. We got off to a late start I think around 4am, the guide was late getting there. We climbed from the altagracia side based on guides recommendation. It was clear at the top and had amazing views. It is exposed, bring enough water. Round trip took us about 7 hours but we were going I think at a faster than normal clip.
Also - make sure to have the ferry ticket sorted out a day before going to the port. It was a zoo when we went, we took our truck and saw lots of backpackers walking around. Not sure how easy public transportation is once you get to the island.