I started running at over 300 pounds (BMI over 45). What I considered a run then was about 10 - 15 seconds running at a 4.1mph, then walking for 5 minutes at 3.3mph. Now at 220 (BMI 33.1), I run 3 miles in just under 35 minutes on an average day. My last long run was 7 miles in 1h25m, I took 2-3minute walking breaks , ran for the rest (between 4.6-5.2mph).