We booked a two hour bike tour with http://www.centralparktours.net/ and it was a great way to see the park. Although my husband had been to NYC on business several times, he'd never had time to really see the park and this was the first trip for the kids (14 & 15) and I to NYC.
Teodor took us on our tour and he was great. He showed us all the major attractions, sights where various movies were shot etc and some little out of the way places as well - where there weren't any other tour guides! He'd been doing tours in the park for more than 4 years so was quite knowledgeable. We actually had one "guide" come up and ask us if the building we were looking at (Dakota) was where Lennon was shot.......so I can imagine there are all sorts of supposed bike tours and "guides" operating in the park.
Central Park Tours have various types of tours (rickshaw, bike, walking, Horse & carriage etc) and you can also rent bikes from these guys as well but for anyone with teenagers, I would recommend the bike tour - it was excellent.
You make several stops along the way and it's not a hard ride at all - so you don't need to be really fit to do it.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.