help with 1.5 mile run (sore, legs, walk)

[jds]

I need help getting a 1.5 mile run in 12:45. I cant seem to get to it. I run sprints of 300 meters then walk 30 seconds and I do 2 miles twice a week. Then I run around 1 mile twice a week. Then I do a little over 1.5 miles and its over 13 mins. I cant seem to figure out how to get to under 12mins and 45 seconds. IT seems that when I have .5 miles to go my lower legs are sore and I just want to quit any ideas on how to get through this?