Get a hear rate monitor and do High Intensity Interval Training (HIIT). It's by far the fastest way to increase your stamina if you're untrained and even if you are trained it's a very effective means of increasing your stamina.

This is backed by science.

While significant improvements in endurance performance and corresponding physiological markers are evident following submaximal endurance training in sedentary and recreationally active groups, an additional increase in submaximal training (i.e. volume) in highly trained individuals does not appear to further enhance either endurance performance or associated physiological variables [e.g. peak oxygen uptake (V . O 2peak), oxidative enzyme activity]. It seems that, for athletes who are already trained, improvements in endurance performance can be achieved only through high-intensity interval training

It is generally believed that in sedentary (VO 2max <45 ml/kg/min) and recreationally active individ- uals (VO 2max ≈ 45 to 55 ml/kg/min), several years are required to increase VO 2max to that of the highly trained athlete (VO 2max > 60 ml/kg/min). [21,42] However, Hickson et al. [43] showed, in eight sedenyary and recreationally active individuals, that VO2max could be markedly increased (+44%; p < 0.05) after 10 weeks of high-intensity exercise training (alternating 40 minutes cycling intervals at VO 2max 1day, with 40 minutes high-intensity running the next, 6 d/wk