This is correct and normal, your aerobic base needs to increase to a decent level, before you can undertake regular exercise and reap its benefits. And it is a painful process, but not a very long one. Six months is perhaps unusual, but it probably depends on your overall health. When I transitioned from being sedentary to daily jogging, I quickly quadroupled the duration I could run uninterrupted in only 4 weeks (from 5 min to 20+ min).