Is yoga enough on its own for fitness?

It depends what you want. Yoga is excellent for mobility, balance, body awareness and, in more vigorous styles, some strength and cardiovascular work. Where it falls short for most practitioners is progressive resistance — bodyweight loading does not increase over time the way weights do, so strength and bone density gains plateau. It also does not build much cardiovascular capacity in gentler styles. A reasonable combination is yoga for mobility and balance alongside two resistance sessions and some brisk walking. As a lifelong practice for joint health and stress it is genuinely hard to beat.