Running: Speed vs Time

jpapa96

I am in my final week of C25K- can't believe I'm actually making it this time! I have been running between 4.8-5.0 mph, and know that in order to progress I will need to both extend the length of time I run and the speed at which I do it. I have freakishly short achilles tendons, so have found that a slower speed makes running much more tolerable. Any recommendations on the best way to push forward? Should I work on increasing the length of time first, or try to do the same length, but gradually increase the speed? I'm currently using a treadmill, but have signed up for 2 5Ks this spring and will be moving outside once the weather breaks a bit more.