I just started running 4 weeks ago. Although I'm not doing any specific program, my pace seems to be similar to the couch to 5k program. I initially was only able to run for 30 secs, walk 2 min...run 30 secs...walk 2 min. and repeat for about 30 min.
4 weeks later, I just RAN my first mile today without stopping. I do a total of 4 miles/6 days per week, with the last mile sprint 30 sec/walk 1 to 1 1/2 min. and repeat.
How I do it, is just going by my polar heart rate monitor and how I feel. I run until my heart rate reaches 175, then when it does I try to slow down my run, but if my heart rate doesn't reduce significantly within 30 sec...I start walking until my heart rate drops to about 120-130 and start running again. This has worked out for me and my running time has consistently increased this way.
Hope this helps...happy running!