I agree with the suggestion of tracking it as 2 entries.
Personally, I use the calculator at www.caloriesperhour.com/index_burn.php
- just enter any two of speed, distance and time (for each of running and walking), and it will calculate the rest.
Presumably from your earlier walking, you have a pretty good idea about your typical walking pace, and you know that you walked for about 50% of the time. Using the link above will automatically calculate the distance for you. You then know that you ran the rest of the way in 50% of the time, and you can use this info to calculate the distance and calories burned running. You canthen enter both of these manually into Spark.
A little bit clunky, but it does take care of all the math for you, and I always like seeing my running speed.