Tamara, although I'm no expert, I think Grog is right -- try the run/walk method to keep your heart rate under control, or slow way down to a jog. Personally, I use the "low-tech" training method for my running -- I just run whatever pace is comforable that day. I know you said you didn't want to add any more running days, but if you're going to get faster and build mileage, you're going to have to run more than 2 days a week. Cycling will complement your running, but to run well, you have to run. I wouldn't worry about running/cycling on consecutive days; I do it all the time. If you feel a little tired, just run/ride a little slower that day.

Good luck!

KB