Keeping my Senior dog living a long healthy life?

I have a Lab/Chow Mix named Mya, and she's getting old, shes around 13/14. But we're not sure. she seems healthy. but lays around alot (but what else can she do?) well i've had her since 2009. And i feed her Purina One. I want her to live for a long time beacause she's been my best friend for so long, and im scared to lose her. so do you have any suggestions for me to keep her living longer? She belongs to my parent, but she does not go for monthly, or yearly checkups, which worries me, but im told they dont have enough time to do so. I really love her and dont want her to leave me, thanks