Many, many of us got back into cycling as older adults, not that 40 is even that old. Quite a few of us have hit higher decades and are still riding strong, and, if you are out of shape, then cycling is exactly what you should be doing! Just have fun riding as a family. Make sure everyone has a well-fitting helmet that stays buckled on their heads, even if it is on a trail or greenway. Greenway asphalt is just as hard as highway asphalt. If you start building up your distance and the bike saddle is uncomfortable, you can change saddles and also get padded bike shorts or a padded liner to wear under your own shorts. What kinds of places are you riding?
Grits
2010 Trek 5.2 Madone WSD, SI Diva Gel Flow
2002 Terry Classic, Terry Liberator