I have to agree with MDHillSlug, based on my own experience: my running speed increased greatly when I focused on running. Running. Running. I could not handle to cycle when I was doing speedwork and long runs, my legs would get toast and it hindered my daily activities. I used to cycle in the winter, but now I dedicate colder days to running, and my bike "training" really only starts after I have completed a spring half-marathon. (Note: I don't do triathlons anymore...)

My first tri I ran a 9-minute mile. My last 8K running race I did a 7-minute mile and finished 5th. I know I will never train enough on that distance to be anywhere near winning that race, which would require a 6-minute mile pace. I trained for running five times a week including a tempo run, intervals, hill repeats, and usually a long run.

I think another part of it is mental. I only progressively discovered how hard I could really run, and how hard I could race. (I already knew how hard I could cycle.) I would do hill climbs trying to crank up my heart rate at every repeat. Crazy stuff. I just didn't know I had it in me to tolerate this sort of extension of myself. I do!

I am sure that you would be able to improve your running speed to the target you hope to reach by dedicating time to it. At first your cycling speed may suffer a bit, but over time you'll definitely be able maintain both in a range that will take you closer to your age group's podium.