A reliable way of gauging your fitness is to measure your hearts ability to recover from exertion.
The Chester Step Test is a sub maximal test which requires you to step onto, and off a 30 cm / 12″ step, at a rate set by a metronome disc, sufficient to elicit a heart rate of around 80% max heart rate at a moderate level of exertion. It is a multi-staged test so every 2 minutes there is an increase in the rate of the step.
Exercise heart rates are then plotted on a graph and an aerobic capacity and fitness rating may be calculated.