I get workout euphoria/runners high about 10-15 minutes into anything that gets my heart rate up significantly. It makes cardio very addictive, but means I have trouble with low intensity work and strength training. Also.. the high is sometimes followed by a low about 15-30 minutes after exercising, although sometimes that is related to refuelling or simple fatigue.
I gradually increased the distance of my walks over the course of like a year and a half. I started off walking to the gas station at the end of the street and back which is just under 1 mile round trip, and now I can do 5 miles 5-6 times a week!